I remember there used to be a countdown timer back in the Flash days, but I'm not sure if it's still there somewhere.
I've had the same experience of wasting my credits without getting to play. In my case, if I sign up early in the lineup, I take my time and play at a more relaxed pace. But if I'm one of the last to sign up, I make sure to play right away.
Anyway, even when I feel like I have plenty of time, I try not to let it sit for more than half a day and aim to play as soon as possible.