Estimates of Time to Learn a Language
I'm a broken record on this, but I'll say it again. Estimates of how long it may take to learn a language are yes, just estimates, so no guarantee it will take you the same amount of time as the estimate. But it's a starting point, a decent place to start guessing for oneself: "okay, if I study this long, I'll be at the skill level to do these X things, because other people who studied this long were able to do X things."
I bring up the estimates to get to B2/Upper Intermediate/Working Proficiency because they're the clearest guess I can find of how long it took some people to get to a place where they could do some stuff in the language they were studying. So for myself, I can go "okay if others took this much time to do those things, I should study at least as long as them before I expect myself to be able to do those things to some degree of skill."
For a language similar to languages you already know, some estimates I've collected are:
FSI Estimates - which are X weeks at 40 hours of study a week (23 hours of classes, 17 hours outside of class). Because they use 'weeks' which assume a lot of study, I prefer to view their estimates in terms of individual hours. Since many people will not be studying the ~40 hours a week FSI expects students to. FSI expects students take classes, so they explicitly study, and they do a variety of practice and engaging with the language. Spanish and French are 30 weeks, 1200 hours total to reach what FSI considers working proficiency which is Interagency Language Roundtable score of 3, which is equivalent to a CEFR B2.
Dreaming Spanish - this website/program conveniently has a roadmap which lists hours. Dreaming Spanish expects students to learn entirely with Comprehensible Input lessons, podcasts for learners in the target language only, and then learning with materials made for native speakers that the student can comprehend. Spanish is 1500 hours total to reach B2 level skills.
Peter Foley - he learned entirely by watching French audio-visual materials. He basically did Dreaming Spanish, but on hard mode, because he used no Comprehensible Input lessons made for learners, no podcasts made for learners, and only French materials. He started with cartoons for toddlers since that was the only materials he could understand at first, then moved onto more difficult materials gradually. He took 1800 hours to reach B2.
For languages very unlike languages you already know:
FSI estimates for languages like Thai, Mandarin, Japanese, for English native speakers would take 88 weeks, so 3520 hours.
Dreaming Spanish estimates 3000 hours. Pablo Roman, founder of Dreaming Spanish, learned Thai through the same method Dreaming Spanish teaches, and it's possible his estimate of 3000 hours was based on the time it took him to learn Thai.
ALGworld (for Thai) estimates 3000 hours to reach the language level of a 6 year old to adult (they make no comparisons to CEFR that I can find)
So in summary, most estimates suggest from 1200-3520 hours to learn a language to B2/Upper Intermediate level of skills. If the language is similar to ones you know, it may take less time. If the language is very different to ones you know, it may take more time. The estimates I found were for both formal explicit study in classes with outside-of-class engaging with the language, and for pure comprehensible input approaches. So no matter how you study, it will take many hours.
(*Yes there are exceptions, if you know multiple languages, it sometimes does take less than 1000 hours to reach B2 skill level in a language. Or if you're very good at making a study plan suited to your particular goals, you could reach particular goals much sooner than 1000+ hours. And then on the other side, there are people who will take longer than 3520 hours to learn a language to B2 - from people who just are bad at making study plans, or have no guidance, to people who will just learn slower. The estimates won't apply to everyone, and plenty of people will fall outside of it. The estimates are likely either an average, or the shorter end of a possible timeline for particularly talented learners. Most people give up long before they reach B2 in a language).
My takeaway from estimates, is that if I've only studied a few hundred hours, that explains why I can't easily confidently do the B2 level skills I am aiming to become able to do. If I want to be able to do B2 level skills, I can see my goal is 1000+ hours of work away, and start planning my study time accordingly.