Every check is a minute.
I might put a check in this box if everyone is well behaved all morning, or if everyone turns in their homework on time, or if you walk to the library and back quietly, or if the cafeteria lady tells me you were really great kids, or just because I like you. You never know how a check might get into the box. It will always be a surprise. I'll never put one in if you ask for one. Once there are a few checks in the box, you get to decide when to spend them.