Generally, you do not want to keep mushrooms for longer than a week. However, the general method of checking that your mushrooms are still fresh is to feel them. If they feel slimy or look darker than normal, it is time to throw them away. I would say about the same if they are stored in their original container or package. If you use a Green Bag, I have kept them as long as three weeks - I'm absolutely serious. If you do not have a Green Bag, a muslin bag or cloth will work nearly as well. The idea is to keep the moisture away from them without drying them out. And if they do become dried out because of lack of humidity, finish drying them on the kitchen counter and keep them in a ziploc bag once they are fully dried for later use.