Ok, I realize the answer is “it depends”, but I didn’t offer that option because I think “about” takes care of that. I ask this question because many learners seem to believe that language learning, hour per hour, is more taxing than work, and you cannot put in the same number of hours without a drastic decline in performance.
I believe the main reason for this is that people do it as a secondary activity, after work or school, where time is limited, and they are already tired from the primary activity. Putting in an hour or two under those conditions is really hard, so when they are asked “what if you had all day free?”, they still think an hour or two is the “limit”.
Another possibility is that they only consider it a hobby; their lives/careers don’t depend on it, so unlike work, they cannot fathom it being done intensively for long periods of time.
I’m retired, motivated, and I have enough resources to learn a language. Once did a 1 year spurt in Korean, averaging about 7 hrs per day. I’m sure there was some drop off in productivity from time to time, but keep in mind that, just like work, activities often change, breaks get taken, etc. My 7th hour was not necessarily less effective than any other hour of the day.
During my 25 year career, I worked many one or more year spurts of 60 hrs/week. My productivity was not lower on the last day of the week, or the last hour of the day. I was not less productive per hour when I worked 60 hrs/week than when I worked 40 hrs/week. Through trial and error, I found out that I wouldn’t want to do any more than 60 though. Based on this, I think I could put in 60 hours per week studying languages and still be plenty efficient.
Even if you have all the time in the world, how many hours you can study without a drop in performance is an individual thing. But if it’s significantly less than how much you can work, imo you are probably doing something quite differently.