Hi Ariff,
The maximum DHCP lease time on RUT routers is 999999 hours, approximately 114 years, which is basically unlimited.
But this is not recommended, because as soon as the DHCP pool runs out of ip addresses, you wont be able to connect any other device in the next 114 years.
If you want to have a static ip, the best thing to do is to configure a static lease in: network->interfaces->lan->static lease.
Best regards.