Some say 2 weeks, others have used crushed grain months old and said the beer came out just fine. Obviously the fresher the better, and it's best to crush shortly before you brew. Just keep it cool and dry.
And taste it. Learn what it tastes like when it is fresh and you will now what it tastes like cold.
__________________ Reformation Brewery: We went professional Obsessing over: starting a local brewery (hence I am not here much these days!), Pickles, my wife, 3 year old and 6 year old
Reading: fall of giants by ken follett
Building: gardens, recipes, and trailer mounted smoker/ wood pizza oven