[citation][nom]visa[/nom]By default I believe it's 45 days in a typical environment (by default). I don't think it's all that tough to have to change your password 4 times a years in order to help ensure network security. At least try to see it from a network admin's point of view. If someone gains access to the network using your account illegally, it's going to be the IT department that will answer for the intrusion, not you. I know a lot of the time IT guys can be dicks but sometimes we have our reasons[/citation]
You don't explain how keeping a password for any length of time is insecure. What is the risk of using a password for more than 45 days, 3 months, or even 1 year? If passwords are insecure after 45 days, they are insecure as soon as you create them. If the risk is people giving away these passwords, that risk is always present, and as gm0n3y mentioned, making people change their password all too often only causes them to use less secure passwords, or worse, write them on a sticky and put them on their monitor.
Minimum password restrictions are one thing (and they are beneficial), but if I already select a 16-character password with a minimum of 2 uppercase characters, 2 numbers, and 2 non-alphanumeric characters, whats the risk if I want to use that same password for a year? I doubt anyone can argue that a password becomes less secure after 45 days, simply because the dangers to password security exist from day 1.
I think password expirations are hold-overs of an old-wives tale. Someone, decades ago, thought it might be more secure to force people to change their passwords. No one ever bothered to figure out if it was actually beneficial or not, but IT still requires it because no one questions convention. Seriously, what happens after 45,90,180 days that makes my password any less secure?