My lessons for each subject throughout the week were 3 one hour lessons, one double period so 2 hour lesson and one 45 minute lesson. I couldn't have sat and concentrated for longer than 2 hours on one subject!
My college has lesson 1 hour 15 mins long - and all 4 subjects everyday. I generally have a free period every day (except Tuesday) and 45 mins for lunch. I find they're just long enough before you get to the "oh my god I'm gonna die if I stay here any longer" phase.
When I was in sixth form, 1 lesson was either 45 or 55 minutes, and a double lesson was about 1 and a half hours. I'd guess that sixth form lessons are shorter as they need to fit around the lessons for the lower school in terms of teacher allocation and room changes. Whereas colleges will obviously focus on 2 year groups.
When I was at college it was 1hr lessons but some science and sport had doubles (for practicals) the 6th followed the school and had 35minute singles and 1hr10 doubles.
at our sixth form each period is 1 hour long, most subjects will have two doubles per week, although they might be with different teachers or one might be a study period. its generally 5 hours of teaching lessons per week and one study period for four a levels, much more independent study for three a levels