There's actually no need to really change the password. The check for used passwords has its limits, so after going through [password]1 to, let's say, [password]20, it lets you use [password]1 again. A script (for loop with smbpasswd, for example) can do this in a few seconds.

My first job out of college had a policy like this, and I did something similar. Biggest problem was that we had multiple password systems, all with their own policies, so my passwords slowly got out of sync.

Mine is somewhere between 30 and 50 days (been keeping track and it seems to shift around). For a while a bunch of us were able to get them to stop expiring our passwords, until upper management found out and put a stop to it.

The counter on my password is somewhere up in the 50s or 60s. (For those of us on linux, it's only used for wifi access - things like email and svn use a different, non-expiring password)