A very relevant factor is that the client starts TRYING to renew the lease when it is halfway through: thus, for example, with a 4 day lease, the client which has LOST access to its DHCP SERVER has 2 days from when it first tries to renew the lease until the lease expires and the client must stop using the network.
During a 2- day OUTAGE, new users cannot get new leases, but no lease will expire for any computer turned on at the time that the outage commences. Another factor is that the longer the lease the longer time it takes for client configuration changes controlled by DHCP to propogate.

A very relevant factor is that the client starts trying to renew the lease when it is halfway through: thus, for example, with a 4 day lease, the client which has lost access to its DHCP server has 2 days from when it first tries to renew the lease until the lease expires and the client must stop using the network.
During a 2- day outage, new users cannot get new leases, but no lease will expire for any computer turned on at the time that the outage commences. Another factor is that the longer the lease the longer time it takes for client configuration changes controlled by DHCP to propogate.