DHSP Lease Times

We are revisiting the DHCP lease times that we use at our site. Currently they are set to 30 days, but we have been running into problems with sites that have Macs issued IP via DHCP. We would like to make the lease time smaller but are not sure what time it should be set at. What are most people using out there?

We also need to have the capability to backtrack IP leases even after they have been assigned to another computer. What is the best way to accomplish this?

TIA