Hi
In the CLI there will be 2 timouts
1) Phase 1 - look for a "crypto isakmp polci "no" lifietime" line.
2) Phase 2 - look for a "crypto map "map name" "no" set security association lifetime" line.
You can modify both but be aware that you would need to modify the other end of your VPN tunnel as well or the IPSEC tunnel might not work.
HTH
Jon