One of my son's flatmates works the equivalent of full time hours over the year. "Active" University teaching is only 2 terms per year, 10 weeks per term, so that's just 20 weeks. Exam season is approximately 4-6 weeks. So basically, they're only actively learning/studying/revising, etc for half the year.
His flatmate works in a supermarket - on a normal/active week, she works Wednesday, Saturday and Sunday, and depending on modules/timetable, she may work an extra shift or two during the week. When she's up against it with test revision, essays, etc she may drop a shift, and she usually takes the 4-6 weeks of exam season off completely.
She works full time in the holidays between terms, being Christmas, Easter and Summer, in fact, some weeks, such as the week before Christmas, she works longer than normal full time, and sometimes works every day, some days double or 1.5 shifts when they're really busy. DS says that over the year, she earns around £15k which is pretty close to "full time" on minimum wage for that age group!
It's enabled her to be self sufficient (parents wouldn't/couldn't pay her any maintenance etc), buy and run her own car, and she's even got some savings.
The thing is, she doesn't go partying, doesn't do sports, etc., so that's a shedload of time that she's got free compared with her peers - she can study/work Friday and Saturday nights, isn't hung over most of the weekend, can work Wednesday instead of doing sports/clubs.
Another of his flat mates works in McDonalds, not quite so many hours, but still pretty substantial, and again, virtually full time equivalent in the holidays.
The time is there for those who wish to, to work quite substantial hours, if they want, and still have time for the studying, but it's at the expense of partying/clubs/sports, etc. The average student will easily spend 30-40 hours per week on those activities. It's just a matter of priorities.