The number depends on your cost structure. I can tell you that where I worked, we traditionally planned on keeping most servers in service for 40-72 months.

When Nehalem processors came out, the economics shifted, and there was a financial case for retiring devices at 24-36 months and consolidating them into virtual machine clusters.

Without that consolidation opportunity, we probably would have extended the lifecycle of our server longer to preserve cash in 2009.