If I pour beer from my kegerator into a pitcher, how long will it stay fresh/good for?

I needed to empty the keg to swap it out for a fresh one, but I still have about a pitcher left, I hate to waste it, so I put it in the fridge with saran wrap over it, how long do I have? Till after I get back from dinner? (couple hours) or till tomorrow afternoon maybe? Or neither? Thanks guys