Hey I learn and maintain 3 different languages! I’ll consider myself qualified enough to weigh in. 
I spent many years improving two of them to a high-level pretty much individually versus leveling all of them in parallel, so I feel like it has given me an interesting perspective on both how learning one language well helps you learn others and how to go about deciding how spend your time.
I think before diving into a new language, it’s important to know what you want to get out of the experience. I personally like to go very deep with the languages I know, and I’ve accepted that requires a long time to realistically accomplish, so that naturally puts a cap on what I learn. There are other languages I’m interested in but I will most likely not exceed 3 until I retire or find myself with a lot of freetime somehow. However, if you are seeking a different experience this will of course change! Maybe you even want to do completely different things with each language - that’s an important thing to know too, because it can guide how much time you devote to each one.
Now, into the specifics of my experience (I shall henceforth refer to the first language learned as L1, second as L2 and so on!).f you add a new language and devote half of your free time to it, that does mean your growth in L1 could end up roughly halved going forward. However, that doesn’t mean the rate of growth for L2+ and beyond will be half of what L1 was. I experienced some efficiency gains when introducing new languages that speed up the acquisition process. I have found these to be dependent in general on how advanced I was in the initial languages when I added others, how close or distant they are from eachother and my native language.
Putting this under a drop-down thing with summary bullets below because wow who wants to read all of this
I learned Japanese first and got to N1 when I started learning German. The experience of getting to N1 kind of prepped me to know what the road to B2 is like (please do not come for me about JLPT levels and CEFR equivalents hahaha 
) and taught me about what I would call the “meta-cognition” about how learning a language in general works. German also happens to be closer to my native language, so those things compounded and I got to B2 in German much much faster than I hit N1 in Japanese. At some point after that, reaching a C1+ level in German became a very big priority for my future goals and livelihood, so I had to evaluate the time I had to spend and deprioritize Japanese to speed up the German growth.
Eventually I hit the level I was aiming for in German and came back to Japanese, and the experience and knowhow I gained from going beyond the B-levels into what I would like to lovingly call “real hard advanced territory!!!” in German transferred back to Japanese and I was able to do what I couldn’t accomplish yet back when it was my only language, aka read a lot of books and understand a lot of native content without issues. In addition, even though German and Japanese aren’t similar at all there has been some language transfer between grammar concepts for me still. Namely, understanding how grammatical case in German has given me sort of a universal understanding of what that grammar does, which has allowed me to see Japanese particles such as を、に、の in a new light (they’re also like cases)!
I finally added Korean after giving my Japanese and German many years to develop and breath, and the fact that my Japanese was already advanced has accelerated my learning like crazy in addition to the other fun gains I mentioned that come just from knowing how to learn. There are so many parallels between grammar and vocabulary in Korean and Japanese that there are constructions or words I’ve only had to learn once and can instantly remember, since I’m essentially scaffolding off the mental framework I have with Japanese already. In particular, advanced vocab in Japanese has been the biggest help because I am quite familiar with kanji and understand how it correlates to hanja - I’ve read experiences from other people who learn both but may be less advanced in Japanese and the gains appear to still be big but not as big.
I guess to summarize the main points about efficiency gains:
- “Knowing how” to learn a language to a certain level will help you learn another one to an equivalent level faster.
- Languages closer to your native language or one you are already quite advanced in are “easier” to learn and will likely go faster or require less dedicated time than L1 did.
- Learning a new language can help you understand how grammar or vocabulary concepts are working on a universal level, which can strengthen your understanding of your L1 in turn.
- Evaluating your short and mid-term goals and adjusting the time you spend on each language accordingly is key. This introduces an element of overhead into the learning process that may not have been there before if you weren’t super goal focused.
In terms of what I’ve had to be lenient with, I’d say the biggest thing is accepting that I’m able to accomplish less in each individual language and being ok with that. I used to read 100+ Japanese books a year and I loved my super reader life and having that much contact with the language. But, my desire to learn Korean was bigger than my desire to keep reading that much, so I compromised. Since I introduced Korean, I read closer to 50 books a year. However, I try to pick ones that seem really interesting to me (if I can, this is sometimes hit or miss) and take my time reading them.
Now I’m pretty happy with the balance between both, but it means I spend zero time maintaining my German, which I made an active decision to be ok with because it is currently less important to me. I would note, though, that the circumstances I learned it under were very different. I spent many years living in a German speaking country and working in the language, so it occupies a very different part of my memory. I don’t feel the need to maintain as much as I do with Korean, which I’ve learned completely outside of the country - the context you gained the knowledge in is also an interesting factor in the amount of maintenance that could be required.
To answer your question about changed viewpoints, this is a little hard because I am actually quite happy with the timing and ways I learned all the languages and have no ragrets! I think it was lucky that the timing coincided with my desire to learn my languages very deeply, because I was not even aware that that was what I really wanted until I was already years into doing all of this. Getting to a high intermediate or advanced state was really the right thing for me before adding more, but I think this is very personal and will differ from individual to individual. 
Like I said, knowing what kind of experience you want to have is an important piece of the puzzle when you are considering adding a new lover…I mean language…to your life.
Current Routine
- Anki in Korean and Japanese everyday, around 100 reps in each. 10 new Korean cards per day, 3 new Japanese.
- 4-5 Japanese books a month (around 1,000 pages)
- Listening Japanese when I feel like it, varies from around 10-40 hours per month unless I am joining a listening challenge
- Korean listening when I feel like it, varies from around 10-40 hours per month
- Learning between 200-400 new Korean vocab words a month using textbooks or from reading books
- 1,000 - 4,000 Korean pages a month depending on my mood and the quality and difficulty of the reading materials
- Having fun everyday!
Edit to add: Certain leverage learning materials in the other languages you know if you are able to find good ones! I have done nearly all my learning in Korean using Japanese resources and us JP ↔ KR for all my anki cards - it means the learning in Japanese does not stop when I spend time on Korean! Definitely my biggest biggest 꿀팁 (honey tip, you all thought you were reading an English post and are suddenly learning Korean now too) for this thread!
, and the bulk of that goes to reading (which, yeah, I’ve indeed read a lot this month so that makes sense).
Right off the bat I think I need to think about my reasons for why I want to learn certain languages, and if those reasons justify a small upheaval in my free time. Having actual, concrete mini-goals to reach the big pie in the sky goal after that would be the next step, potentially. Like I said, much to ponder over. 
