GOAL or METRIC: Refresh servers after four years
RATIONALE: Starting in the fourth year of the life of a server, support costs increase dramatically: about 40% more in year four, 200% more in year five, 270% in year six and about 400% in year seven. Best-of-breed organizations use a manageable refresh cycle of every four years (begin retiring at year four) and/or 20% hardware refresh per year (20% of the data center replaced each year).
SOURCE: IDC: Managing IT Infrastructure Renewal: A Business Framework to Reduce Server and Storage Costs: September 2010
Its hard to get management to follow a 5 year SDLC though when budgets are cut and you are looking at layoffs.