I've only prestiged once or twice in Modern Warfare 3 and it's taken me hours and hours. As a social game, with my girlfriend on split-screen and other couples over the internet, Call of Duty has truly become unmatched but I suspect the people at the top of the leaderboard are unlikely to be playing with their friends and relatives.
For those people: good news. Infinity Ward have increased the amount of prestige levels in the game. There are 400 new levels to get and five new times to prestige: that's probably a few day's work for even the best of the best. This is currently patched into the PC version of the game and the console version of the game will be updated in the not-too-distant future.
The update has been met with mixed reaction: people seem upset that this update, which was probably as easy to implement as changing a couple of numbers and hitting enter, took precedent over fixing lag issues.