This is directly related to my question regarding leasing or purchasing in flights versus piecemeal design: http://serverfault.com/questions/18777/switching-from-piecemeal-machines-to-leases
When you buy a machine for a user, how long do you assume it will be in use? Do you plan 3 year or 5 year generations, or something else? Is it different for laptops vs desktops? (or are you even still buying desktops? I've bought 4 in as many years, I think).