I definitely dont think that there should be a limit on how many games you can rate in a day, unless it is rediculously high... I play a ton of games each day when I get in a groove, and rate all of them.
I do think that the idea of having to at least be on the page a certain amount of time before rating a game is a good one, though it should be at least a minute. If you havent played the game for at least a few minutes, you have no idea what youre rating. This would also slow the ability to rate a game without playing it just to rack up points.