Humidity and the quality of the leather used in constructing the holster would be two big factors.  I have seen some revolvers from the east side of Montana which were stored in leather holsters for periods of years look like new when removed from the holster.  I have also seen some come out of a holster that made you feel sick just looking at them.  I store mine on rubber coated steel racks in the safe.  The one that's not in the safe is stainless, is loaded and kept in an El Paso Saddlery holster.