I make a small batch (100 to 200g) so it doesn't last very long and I am not sorry if I have to throw it away. As long as it is in a spray bottle that was sanitized and kept refrigerated I would think it could be effective for a month. Most people just get rid of it when it smells. The spray is usually very effective. If you sprayed cheese 3 times within a week and PC is not blooming nicely than your culture is most likely dead or lost most of its effectiveness. I don't keep it that long because usually the next cheese will be some other type and I don't want to fill the fridge with bottles of aging cultures...
