When do you think it'll happen?
GTA V was announced in late 2011 - a little more than 3 years after IV's release, and almost 2 years after TBoGT's release.
GTA IV was a large success, but V was an even bigger one. R* may say things about conserving their energy and keeping GTA interesting, but let's face it, companies care about money a lot. Something tells me they'll be eager about making a new installment of a popular, hot-selling franchise even if they're sitting on top of a lot of money already. And with the PS4 and Xbox One having more power, it'd be a good way to show off.
So I'm betting on an announcement sometime in 2016, maybe next year if we're lucky and the recently started 8th gen of gaming takes off well (they need a market after all). According to other announcements by R*, we can expect at least one other game before that (hopefully Agent).