As far as how many years, I believe they are looking for how long you have lived there since you moved back. I also don't think it really matters all that much (for this purpose, anyways) so I wouldn't worry about whichever one you go with.
However, with moving back specifically for college, i'd just make sure you read up on whatever state regulations your schools abide by, make sure you meet them, and then prepare all documentation to prove it. I was in almost the exact same situation as you receiving in-state tuition etc and still initially got denied in-state residency at one of my state schools I applied to (they even cancelled my interview, it was a nightmare). Had to appeal it and provide lots of paperwork to get that decision changed.
As long as you meet the regulations for your state, you should be okay, just be prepared to prove it (and hopefully it's not necessary, but better safe than sorry).