DHCP Server must delete the "expired records" from its database (dhcp.mdb file) so that it releases space for new IP Addresses. By default, it does every 4 hours. You can modify this value by hacking the following registry entry:
- KEY Name: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DHCPServer
- Entry Name: DatabaseCleanupInterval
- Type: REG_DWORD
- Value: (in minutes), default is 4 hours.