The MX Security Appliances and MS Layer 3 Switches have a DHCP feature that controls IP Address leases and lengths. DHCP leases can be set for varying periods of time up to 1 week. Client machines will renew their lease early, which is expected behavior.
The client first attempts to renew its DHCP lease after half of the lease time has elapsed. The client will send a DHCP Request to the DHCP server, requesting the same address the client currently has. This will be recorded in the Event Log as a DHCP lease.
A more detailed explanation of DHCP lease times and renewal behavior can be found here, at The TCP/IP Guide.