Chapter 10 Install and System Administration for FortiOS 5.0 : Advanced concepts : DHCP servers and relays : Lease time
  
Lease time
The lease time determines the length of time an IP address remains assigned to a client. Once the lease expires, the address is released for allocation to the next client request for an IP address The default lease time is seven days. To change the lease time, use the following CLI commands:
config system dhcp server
edit <server_entry_number>
set lease-time <seconds>
end
To have an unlimited lease time, set the value to zero.
See Also
DHCP Server configuration
Service
DHCP options
DHCP Monitor
Breaking a address lease
DHCP servers and relays