I've stored mushrooms for 4 years with no noticeable loss of potency (just confirmed the potency of this batch by microdosing 0.3 grams).  The fresh mushrooms were semi-dried for 1-2 days with a fan (no heat), then fully dried using activated alumina.  I store them in the dark, at room temperature, in a sealed plastic container with a lot of excess (orange bead indicating) silica gel packets.  The beads are always orange when I check on them, so I know little to no moisture is being absorbed from the mushrooms or the environment.