How often should PC's be replaced? IT Managers Respond

Here's an interesting article from ComputerWorld, PC upgrades: When to turn the page isn't an easy call for IT execs. 

When I was in consulting working on deployment projects, it was always interesting to me how companies decided upon the lifecycles of their PC's and laptops.  Obviously different customers had different opinions, and as you can see from the article that's still the case.  Usually desktop lifecycles were a year or two longer than laptop lifecycles, and experience says that makes sense. 

One lifecycle policy I hadn't seen before was the Virgin Entertainment Group "on-demand" refresh policy, (I think the guy on the left is taking "on-demand" a little too literally, although that is how I talk to my laptop sometimes J).  I'm sure "on-demand" refresh saves a bunch of money on actual PC costs, but I wouldn't want to be on the support end of it as it's got to be a nightmare supporting so many different desktops.  Surely any savings made are canceled out by increased support costs.  Maybe not though.  Now I think about it, a lot of desktops are single application workstations, so if the app is running OK why replace the desktop?  So glad I'm not responsible for making the decision J. 

As a laptop user for the last 15 yrs I do think that trying to keep a laptop more than 2 yrs is tough.  Maybe its because I'm a road warrior and they take quite a beating (thanks TSA J) but when I turn mine back in after 2 yrs for refurbishment, there's quite a lot of work to be done on it to bring it up to scratch. J

Personally, I think 2yrs for laptops and 3-4 yrs for desktops are a good rule of thumb.  But are they the right numbers?  Who knows, I just picked these arbitrary numbers based on some experience, but what's right for some might not make sense for others, but I think they're a good place to start.

What do YOU think?  Too long?  Too short?  Or in your case you don't care because you get yours replaced whenever you want? J