When is a MacBook Pro considered at 'end of life' and in need of replacement?
In a typical corporate scenario you have to balance cost, resale value, and depreciation. This is all the accounting department really cares about, and that's who the IT department reports to.
In general terms:


Macs are more expensive than the equivalent PCs. In other words if you need to do X, Y, and Z and it can be done on either platform, the cost of the minimum machine that will do it is less for PC than Mac.
Macs have a high resale value. After 2 years a PC will have dropped to a very low cost, but a similar Mac will still hold significant value.
Macs depreciate slowly, while accounting typically uses one depreciation value for all computer equipment.


The combination of high resale value, but the same depreciation may tip the scales in your favor - or at least temper the initial cost of the machine. The machine may be partially or fully depreciated when it's sold, but it will sell for a higher cost than the PCs they would sell at the same time.

In most scenarios anything shorter than a 2 year replacement plan is going to cost the company more money than the value they'll get out of the machine, but anything longer than a 3 year plan will cost the company in terms of performance (employee waiting for the machine to catch up) than they'll lose buying a new machine and selling the old one.