if you use Microsoft DHCP, you may have seen that lease last longer than expected. Between 4 & 5 hours more.
This comes from the grace period, which last 4 hours by default. If your dhcp is quite full or having people moving around, you may reduce this grace period:
Value Type: DWORD
Value Data: time in minutes
So that explain the 4 hours. But why 5 ? Because the dhcp cleanup only occurs every 60 minutes by default, except if dhcp lease are starving. You can make it more aggressive:
Hope you will find it useful!