Hey there,
I think you'll want to set the isakmp lifetime to 0 per : http://www.cisco.com/en/US/products/ps6120/products_tech_note09186a00807e0aca.shtml#solution06
Specify the SA lifetime. This examples sets a lifetime of 4 hours (14400 seconds). The default is 86400 seconds (24 hours).
PIX/ASA
hostname(config)#isakmp policy 2 lifetime 14400
IOS Router
R2(config)#crypto isakmp policy 10
R2(config-isakmp)#lifetime 86400
If the maximum configured lifetime is exceeded, you receive this error message when the VPN connection is terminated:
Secure VPN Connection terminated locally by the Client. Reason 426: Maximum Configured Lifetime Exceeded.
In order to resolve this error message, set the lifetime value to 0 in order to set the lifetime of an IKE security association to infinity. The VPN will always be connection and will not terminate.
hostname(config)#isakmp policy 2 lifetime 0
Good luck!
Tim