Unfortunately, the general answer to that question is "a while".
Unlike games, which release at the peak of their price, movies tend to be sold week of release at good prices. After that first week, the prices tend to go up, much closer to MSRP, and don't tend to fall for quite some time. They are also usually cheap in Q4, but we're at the end of Q4, so unless someone has a surplus of recent releases, you'll probably find that the prices will hold for 3-6 months.
Arguably, Video Games should be sold with a similar price model, with a significant discount to those that pre-order the game, and the recent sales on Modern Warfare 2 and Halo ODST on Amazon (and other retailers) are showing the first signs that many other game sales may soon follow. iPhone games are already selling that way, and it won't surprise me if most game manufacturers decide to follow (instead of having the usual Ubisoft $20-$30 price cut after 2 weeks).
But as for the question at hand, with new movies you should look for deals week of release, and if it's a movie you really want, remember that it could be a while before it comes down in price if you pass up the initial sales.