Kittens, who should be given smaller meals more frequently, probably will finish their meal well before 15 minutes. It's better to give them only a tablespoon or two every three hours than to take a chance on spoiled food.
Canned food remaining in the can should be covered and refrigerated immediately after opening, and the next serving can be warmed, as most cats don't like cold food. I sometimes use zippered disposable bags for storing uneaten food. They can be readily warmed with running water in the sink for a few minutes. An alternative would be to put the next serving in a microwavable dish, cover with plastic wrap, and warm on a low setting for a short time. You don't want to heat the food, but warm it to room temperature.