Yes, you need the Domino Designer client to modify that. Go to the Agents, open the Out of Office agent, open the Properties window, and modify the schedule. It is a per person parameter, unless you modify the template and propagate the changes to everybody's mail database.
You can also move to R8 or R8.5, because the OoO functionality has become a service on the Domino server.
By the way, where does DHCP come in here?? ;-))
