Datacenter servers are no different from servers used in Corporate Environments. You can still find old Dual PII 350MHZ servers in production if you were to dig around due to companies not wanting to migrate the data to a new server.
The "physical" and "hardware" lifespan though is different than the retail lifespan of the server, which is actually decided by the clientèle.
Where one server might be purchased and able to be leased for three or four years before the income variable on the server makes it an unfavorable option for resale another server might be purchased and leased to a client for 8 years before they are ready to migrate there clients or data off of it.
There is no straight way to figure the actual life, but I would say two to four years on average.