From Longman Dictionary of Contemporary Englishbefore you know itbefore you know itused to say that something happens very quickly and when you are not expecting it You’ll be home before you know it. → know
Examples from the Corpus
before you know it• Spring break will be here before you know it.• You'll be fully recovered before you know it.• It happens before you know it.• Take time with people, and you will see success in your business before you know it.• They claim that news is just around the corner, and that it will be on us before we know it.• One thing kept leading to another, and before I knew it a small industry had been set in motion.• You offer to iron his shirt and before you know it, he expects you to do all the housework.• He saves his money, before you know it he owns a car.• It seemed a long way away but before we knew it we were paying the last of the deposits. 