The Parker O-ring handbook is a good reference for these sorts of questions:
7 Storage
The effective storage life of an O-ring varies with the
inherent resistance of each individual elastomer to
normal storage conditions. ARP 5316 places elasto-
mers into three groups according to “Age resistance
generally associated with products fabricated from
various rubbers.” Realize that this document, ARP
5316, is an Aerospace Recommended Practice, not a
standard that must be met.
Where non-age sensitive elastomers are involved,
considerable storage life without detectable dam-
age is common even under adverse conditions. For
materials falling into the 15 year category, which are
subject to age deterioration, the following conditions
are suggested for maximum life:
1. Ambient temperature not exceeding 49°C (120°F)
2. Exclusion of air (oxygen)
3. Exclusion of contamination
4. Exclusion of light (particularly sunlight)
5. Exclusion of ozone generating electrical devices
6. Exclusion of radiation
Generally, sealed polyethylene bags stored in larger
cardboard containers or polyethylene lined craft paper
bags ensure optimal storage life. However, in normal
warehousing conditions, life of even the relatively
age-sensitive elastomers is considerable. This is due
to major improvements in modern compounding tech-
nique, and has been documented through a number
of investigations concerned with effects of long-term
storage of elastomeric materials undertaken in the
recent past. These include controlled laboratory stud-
ies of many years duration in addition to evaluation of
seals recovered from salvaged World War II aircraft
and other sources after exposure to widely varying
conditions over many years.