As soon as you check in, your credits are available to use. You can use them however you want until midnight of the day you check out.
You don't have to use 1 each day. You can do two TS meals in one day and do two QS meals on another day. You can use all of them in one day, but when they're gone, they're gone.
Since you're arriving late in the day, I'd probably plan on a QS at my resort that evening.