Put it simply, once you attend your first class at the college level (Juco or 4 year school) your 5 year clock starts. From that point, you have 4 years of eligibility. If you're just a regular student and you decide to walk on playing football after you've already been in school for 2 years, you only have 3 years of eligibility left.
I believe that if you go to school for one year and then drop out, your clock is still running. For instance, Chris Strong went for 1 year and flunked out. He used one year of eligibility this past fall. This fall would be year 2, and he's obviously not going to play anywhere. So hypothetically, if he got his academics back in order by fall of 2009 he'd be able to return with 3 years to play 3. However, if he waited until fall of 2010 and for some reason got academically eligible, he'd only have 2 years of eligibility left.
To answer though, it doesn't matter what sport you play, you have 4 years of eligibility to use in that sport. If you played baseball your first year in school, quit and then decided to play football, you'd still have 4 years of football eligibility, but you'd have burnt your redshirt year playing baseball.