Those messages are harmless, they're just letting you know that the firewalls are doing what they're supposed to do and rekeying the IPSec tunnel periodically (default 1 hour).
If you really want to increase the IPSec SA lifetime, it's done in the Phase 2 Proposal on the SSG. The screenshot you sent has an arrow pointing to Dead Peer Detection / Hearbeat settings, which are Phase 1 settings.
You'll need to create a custom Phase 2 proposal (VPNs -> Autokey Advanced -> P2 Proposal) with the SA lifetime you want there. Then you'll need to set your VPN (VPNs -> Autokey IKE -> your VPN -> Advanced) to use the custom Phase 2 Proposal that you created.
You'll want to make sure that the Linksys is set for a Phase 2 lifetime of the same value you set on your SSG. Often times those smaller firewall/routers don't separate the terminology and they confuse Phase 1 and Phase 2 settings. Phase 1 and Phase 2 have independent lifetime settings.