Dan,

The quickest way to get there is probably 'show run | in crypto ipsec'. The command you are looking for is 'crypto ipsec security-association lifetime'.

If you do not see it configured, then your ASA is using the default time of 28,800 seconds (8 hours).

'crypto ipsec security-association lifetime' Command Reference:

http://www.cisco.com/en/US/docs/security/asa/asa80/command/reference/c5.html#wp2190165

Hope that helps.

-Mike