To define appropriate values for lease duration, you should consider the frequency of the following events for your network:
All of these types of events cause IP addresses to be released by the client or cause the leases to expire at the DHCP server. Consequently, the IP addresses is returned to the free address pool to be reused.
If many changes occur on your internetwork, you should assign short lease times, such as two weeks. This way, the addresses assigned to systems that leave the subnet can be reassigned quickly to new DHCP client computers requesting TCP/IP configuration information.
Another important factor is the ratio between connected computers and available IP addresses. For example, the demand for reusing addresses is low in a network where 40 systems share a class C address (with 254 available addresses). A long lease time, such as two months, would be appropriate in such a situation. However, if 230 computers share the same address pool, demand for available addresses is much greater, so a lease time of a few days or weeks is more appropriate.
Notice, however, that short lease durations require that the DHCP server be available when the client seeks to renew the lease. Backup servers are especially important when short lease durations are specified.
Although infinite leases are allowed, they should be used with great caution. Even in a relatively stable environment, there is a certain amount of turnover among clients. At a minimum, portable computers might be added and removed, desktop computers might be moved from one office to another, and network adapter cards might be replaced. If a client with an infinite lease is removed from the network, the DHCP server is not notified, and then the IP address cannot be reused. A better option is a very long lease duration, such as six months. A long lease duration ensures that addresses are ultimately recovered.