It seems that most of the computers have a life of 3-4 years before the new models are to good to resist. The only time I strayed from the 3-4 years for me was when We went INTEL.
many firms have a 3- 5 year depreciation schedule if they try to run up-to-date software at decent speeds
i remember holding out for a G5 laptop ... and then intel came and changed the whole game