Setting DHCP lease times is in practice pure voodoo. I ran across a paper presented in 2007 on research into DHCP lease times at Georgia Tech’s student network called Usage-Based DHCP Lease Time Optimization.
While the paper on the whole is interesting, what caught my eye was the exponential lease time experiment, and how well it fit GT’s DHCP usage. Instead of a fixed-length lease time, the initial lease is 30 minutes, with each renewal doubling the lease time (up to a maximum of 30×25=960 minutes). Those machines connected for about 8 hours received 16-hour leases, allowing them to avoid reconnection (and reäthentication, which is a bother for users).
This exponential approach brought to mind what I learned in my operating systems class (waaaaaay back in the 1980’s), where the probability of a process terminating within the next fixed unit of time decreases exponentially with process run time.