To change the DHCP lease time you will need to telnet to the router’s Command Line Interface (CLI).
To check the current settings enter the following command:
srv dhcp leasetime ?
Default setting is 259200 seconds (3 Days)
To change the DHCP lease time enter the following command:
srv dhcp leasetime <Lease Time (sec.)>
For example to change the DHCP lease time to 24 hours (86,400 seconds) enter the following command:
srv dhcp leastime 86400