Before you know where you are
Definition 1 of 1
Pro Tip 1/3Surprise Element
The phrase conveys an element of surprise regarding how quickly something happens. 
I just started college, and before you know where you are, it's already finals week!
Pro Tip 2/3Informal Speech
It's commonly used in casual, conversational English rather than formal situations. 
We'll start the party, and before you know where you are, it'll be midnight!
Pro Tip 3/3Time Flies
The phrase can emphasize how time seems to pass very quickly. 
The kids grow up so fast—one day they're crawling, and before you know where you are, they're off to school.