Teaching and preparation don't take up that much time, but the big time suck is grading. At my very best, I can get about 4 papers graded and commented on per hour. Usually, it's more like 3 an hour. If you have four classes of 25 students, that's 33 extra hours for every major assignment. On a semester system, you're likely to have at least 4 or 5 papers, plus smaller assignments. Adjuncting at 3\3 was often a 40 hour \ week job for me, sometimes more, and I was making considerably less than what I've received for non-teaching jobs.
-- Best,
Jordan