Well Damsel that depends a lot on which species it is... as for the species I keep, Sepia officinalis, They should live for about 18 months or so in the wild.
The one I have is now about 9 months old and very obviously sexually mature.
I have him at 72 deg F and that is towards the warmer end of his spectrum. it is a case of live fast die fast if you keep them in warmer water. If I had a chiller and kept it at 60 deg F or so he would live longer...
Okay, i'll guess at this one living 12 months or so, but I'm not sure yet. his mate lived 8 months but that's cuz he mated with her and then killed her!
C