About as long as milk does.

Ford

It will change it's flavor as it stands in the cold, usually getting sweeter.  

Mini

I always use it by the next day.  I've been happy by using a poolish to start my potato bread overnight, so if I'm using the potato water from cooking potatoes for dinner, I let it cool to lukewarm, then make my poolish with it that night and bake the next day.

Thank you so much, I will toss it, it is a week old.