The lease time means life of an IP address remains assigned to a device. Once the ip address life ends, the address is released for the next request of an IP address.
The default lease time is 7 days.
`To modify the lease time, use the following CLI commands:
Step-1 Switch to configuration mode
system config dhcp server
Step-2 show the list of available dhcp servers type
get
Step-3 chose the dhcp server you want to edit in my case server entry number is ‘1’ so i will type
edit 1
Step-4 set the lease time according to your requirements, i will set lease time ‘1’ hour  (1 hour = 3600 seconds).
set lease-time 3600
Save the configurations
end
That’s it, you have successfully reset the dhcp lease time on your Fortigate device.