you snooze, you lose
If you wait too long to do something, that opportunity might become unavailable. The phrase can be used as a warning for events to come, or to describe past things that one has already missed. Come on, we have to get to the bakery early, before all the donuts are sold out! You snooze, you lose! A: "Aw man, I can't believe I missed the chance to meet my favorite author." B: "Well, you snooze, you lose."