Hi Adam,
Please check up below command, it's been verified with 100.X version.
Usage: set dhcps {options}
-> {enable|disable}
-> iprange
-> leasetime <90..86400> (in seconds)
-> example: set dhcps leasetime 90 (please reboot AP to make this take effect!)
You could reduce leasetime 90sec but I do not recommend too much low lease time interval.
Thanks
Chang