In DLI we did 7-hour days, based on 7 x 50-minute class sessions. We didn't necessarily get up and switch classes each 'hour' (unless going to the lab), but we did change subjects or modalities regularly. Throw lunch in there to split the day. PT and dinner after class. And of course the hours and hours of homework at the end of the day. Your day was pretty much full from 0500-2200, roughly.
My refresher class at
FLTCE in 1990 was 7-weeks long in a class of about 5-7 people. It was much like DLI as far as the variety of teachers and subjects, but included one morning per week a 1-hour one-on-one speaking session with a teacher. It also included a daily, one-hour Russian grammar class which was taught in English. My Reading and Speaking skills went up two steps (1 to 2) and my Listening went up one (1 to 1+) at the end of the 7 weeks. And that includes a wasted first week of class as it coincided with Oktoberfest.
My few one-on-one sessions (2009 and 2014) throughout my career were for no more than four hours per day, and again generally on a 50-min class length, with a change of subject or modality. This would be followed by 3-4 hours of homework to complete the day. My only issue with these types of class (and the longest of these I've ever had was 4.5 weeks long) was hearing the same voice every day. I improved my proficiency levels in all 3 testable skills (listening, reading, speaking) by one step at the end of each of these sessions (2009: 2L/2R/1+S to 2+/2+/2; 2014: 2/2/2 to 2+/2+/2+).
This isn't an answer to the OP but to smallwhite. In all these examples, I'm receiving instruction. I can study a language almost any amount of hours per day if I'm getting instruction. When I'm studying on my own, my diminishing returns starts at about one hour, depending upon what skill I'm working on and if I'm working on the language straight through vs. breaking it into 15-min segments throughout the day (like my commute).