Hi Zen2000.
My experience is based on O-rings made from black NBR materials.
These compounds are sensitive to sunshine ( among others )which will reduce their flexibility ( cracks ).
Storage time for a couple of years are possible to my experience if they are kept in a dark area, but you'll see some of the compound elements migrating to the surface over time
Ambient temperatures (Europe) of 20-25 deg. Celcius are acceptable.
rgds.