The long time will have a side effect: If I update the port's IP via API, and the VM will not be updated until the long timeout.

@Yong sheng

i agreed.

and it can be affected to live migrated vm ( like some one using multiple gateway and zones, it will take time to notice the gateway ip is changed by dhcp timeout)

The lease time was low because previously the neutron-dhcp-agent didn't have a way to release the lease until it timed out. In the havana release the lease time is set to 24 hours though and the ip address can be reclaimed after a port is deleted.