How often do you replace company’s computers?

We have about 70 computers on the LAN where some are as old as 7 years old (yes, it’s that sad..). Since money is tight, we only replace the computers that are no longer working. The drawback of this is that some of those that are still “dreading” are creating problems such as locking up. Due to this I’m hoping to learn what is the norm of replacing/upgrading computers at other work place. I’ve heard 3 year’s the average time frame. What do you do with those machines that have reached that age but are still in fairly good condition? Do you replace them anyway or give it to light users?

Any input is greatly appreciated.

Thanks.