Dried shrooms last up til 6months, but you should keep them conserved (jar) and in a dark cool area. In Christiana they had shrooms so dry it was like powder and chunks, we bought some and there was no problem with the effects. Like after 6 months or so they decrease in strength but if your shrooms are not that old then there should be no problem even if its like a powder, as long as you have the right kind
3 weeks goes for fresh shrooms (in fridge) after that they kinda rot very easily.