A lot of colleges let you take any course you want, regardless of your degree. There may be prerequisites for the courses (ex., Calculus generally requires you to have taken Algebra first). They get money from you per class, and the more extra classes you want to take for fun, the better for them.
As far as class lenght, depends on if said college is on quarters, semesters, etc. Around these parts most are still on the quarter system. Altho from what I understand many school across the country tend to do semesters. You can always get ahold of an advisor (not always and easy task but worth the effort). They can, well, advise you pretty well.