Andrew Luck apparently indicated on KNBR recently that he wants to make sure and get his college degree before heading off to the NFL:
"I don’t think about [the NFL Draft] that much," the redshirt sophomore said on KNBR. "I still consider myself a pretty young guy. I need to get a degree in college before I start thinking about that stuff. I will probably stay in school for sure."
Some folks might think that means he'll use up all his eligibility at Stanford and stay into this senior season. However, given his status as a red-shirt sophomore, that doesn't seem likely. Or at least, not likely if he's reasonably smart. As a redshirt-sophomore, Luck could stay at Stanford five years if he so chose. There's a good chance he'll finish up his bachelor's degree by the end of his red-shirt junior year, which would put him at four years of college.
Some players have started masters work and stayed the fifth year, but given the amount of money that could be on the table if Luck continues to progress, it'd be tough to stay one more year and risk injury after he already has his bachelor's degree.