I believe you have until at least midnight the night you check out to use DP credits. Basically you are given a pile of DP vouchers (virtually that is), and each time you use credits, you are given a receipt that lists how many you used and how many are left. I don't think the restaurants pay attention to the dates you're there, but only at your number of credits remaining. So you'll want to make sure you get enough DP credits to cover all the days you wish to be there.