It usually depends on your capital depreciation cycle as well as your budget. Of course, you also need to measure how much power do you really need per position.
For power users/developers/similar high-end users, anything that makes them significantly faster is a worthwhile investment. Then you balance that with how often is that capital investment going to be recouped. For high-end users, usually 2-3 yrs is a good balance.
For regular users (main office productivity types), I have found 4-5 years to be a decent replacement interval.
For servers, their replacement is usually need-dependent. If your Exchange server is slow & dragging, obviously it should be replaced sooner than later. However, due to their (usually) higher-end hardware, they may suit their purpose for longer periods of time. Usually I can get 4-5 yrs from them.
iPads/iPhones are usually replaced after 2-3 yrs, depending on how much abuse they've received.