I agree with Ken's advice to prepare for a backup career, and wanted to directly answer your question about how long you'd be in college before being drafted.
You have to have been out of high school for at least three years to enter the NFL draft, so many players spend those three years in college. Most players don't enter the draft until four or five years after high school, when they've completed their degrees (this also helps them be prepared for whatever their backup plan is). Also, remember that you can declare for the draft, but not get picked and then not be able to play in college any more, so if you leave college after three years to be drafted, you want to be sure you're good enough to be drafted. This would be something you could discuss with your college coach and your family.
Here is more info about the draft: http://operations.nfl.com/the-players/the-nfl-draft/the-rules-of-the-draft/