Most enterprises use DHCP to lease out IP addresses to computers on their networks, and DHCP lease duration can be configured on a per-scope basis using the DHCP snap-in. The default lease is 8 days, after which a computer has to renew their use of the address they've been leased by your DHCP server. There are certain situations however when you might want to lengthen this lease period to several weeks or months or even longer. These situations include (a) when you have a stable network where computers neither join or are removed or relocated; (b) when you have a large pool of available IP addresses to lease from; or (c) when your network is almost saturated with very little available bandwidth and you want to reduce DHCP traffic to increase available bandwidth (not by much, but sometimes every little bit helps). It's generally a bad idea to use unlimited leases however, as eventually you'll move a computer from one building to another or replace a network card or do something else that requires gaining a new IP address.
Mitch Tulloch is President of MTIT Enterprises, an IT content development company based in Winnipeg, Canada. Prior to starting his own company in 1998, Mitch worked as a Microsoft Certified Trainer (MCT) for Productivity Point International. Mitch is a widely recognized expert on Windows administration, networking and security and has written 14 books and over a hundred articles on various topics. He has been repeatedly awarded Most Valuable Professional (MVP) status by Microsoft for his outstanding contributions in supporting users who deploy Microsoft platforms, products and solutions. Mitch is also a professor at Jones International University (JIU) where he teaches graduate-level courses in Information Security Management (ISM) for their Masters of Business Administration (MBA) program. For more information see http://www.mtit.com.