Depends on the type and the current batch. Hellfire used to expire quickly, like a month and a half or so, whereas Marbs would last for 2 1/2 months just fine. Right now the runs of Powerball I get last about 2 months if I stir them twice a week or so. Allstar is really less than that, about five weeks without any apparent negative effects. Hellfire right now is excellent at two months. I keep all my paint on the top shelf of my closet.
All of this could be subject to change. Paint changes slightly all the time.