I am assuming you are on the dining plan. I'm also going to assume it's the regular dining plan (1 TS, 1 CS, 1 snack per night).
You get credits based on how many nights you stay at the resort. So on the regular plan you have 8 TS, 8 CS, and 8 snacks per person. As long as you're still staying at a Disney resort it makes no difference that you are going to Universal, you still get credits for 8 nights. You get all those credits as soon as you check in and you have until midnight after you check out to use all of them. You do not have to use them on particular days.