I really think it depends on the developer. Some like Evolution, Media Molecule, Eat Sleep Play (2nd party) and Incognito clearly need more time. While others Like Naughty Dog, Guerrilla Games and Sucker Punch and Santa Monica only need a bit more flexibility sometimes. Kinda agree with Rol. If SONY allows devs that need extra time to get that time required then there's really no problem. With that said, of course SONY has to draw the line somewhere. So perhaps a 2.5 year average and 3 if the devs are looking to build a new engine from the ground up (which is usually the case for moving on to the next system).
EDIT: When I think about it, I honestly think time wasn't that big of an issue for some of these developers. Just look at Team ICO. Their highest selling game has only sold about a million but SONY's fine with allowing those guys 6+ years on the next game. Granted, their games are very massive but I think it shows SONY isn't exactly the strictest publisher around and care about critical success as well as commercial (i'm assuming they get this freedom because of the critical success of past games).
4 ≈ One
