I've always pre-ordered games well in advance so I'm not too sure, but I think Amazon's pre-order credit bonuses are still there until the day the game releases (possibly ending the day before).
I'd recommend to just pre-order Reach now if your worried about them ending the pre-order credit bonus. Amazon does not charge you anything until it ships. If you choose to get Release Date Delivery, expect to get charged usually 1-3 business days before the game is released.