Hi Melanie! You can use your quick service credits whenever you want.  For each night of your vacation, each member in your party will receive two quick service meals.  You can choose to have a meal for breakfast, lunch or dinner and the timing is up to you - you can use them anytime during the restaurant hours.  

We typically use a snack option for breakfast.  If we are going to Magic Kingdom, we wait to use our snack option at the Bakery as they have some yummy snack choices on their menu.

I know the dining plan can be tricky at first - please let us know if you have any follow-up questions.  Diane