From Sabato's primer on convention bounces:
Bounces oftentimes fade by Election Day. Jimmy Carter slid from 62% after his convention to 50% on Election Day 1976; Michael Dukakis from 54% to 46% in 1988; and Bill Clinton from 56% to 43% in 1992. Meanwhile, George W. Bush lost six points from the end of the convention to Election Day in 2000, and John McCain lost three points.
Gallup also uses history as a guide:
Everything else being equal, Romney and Obama could each expect to see a five-point increase in his support after the coming political conventions, which could change voter preferences at least in the short term. Gallup Daily tracking has shown Romney and Obama generally within two points of each other in registered voters' preferences. Thus, if Romney gets a typical bounce, he could lead Obama after the GOP convention, although Obama would essentially erase that lead if he, too, gets a similar bounce. If one of the candidates gets a bigger bounce than the other, that candidate could well establish a lead in the race after both conventions are over.
The Dish 