I've often struggled with a hard and fast policy for replacing laptops in a business environment. Some people swear by a 3 year rotation, some go up to 5 years. Some use equipment until the old equipment wears out. I've decided that this is what I'm going to recommend to my clients:
Laptops with conventional hard drives - replace after 3 years
Laptops with SSD hard drives - replace after 4 years
Desktops (aggressive) - replace after 4 years
Desktops (conservative) - replace after 5 years
This is as much about likelihood of failure as speed after a certain amount of time.