In the business world, the going rule of thumb is that PC desktops get replaced after every 4 years and PC laptops get replaced after every 2-3 years. Based on my experience over the last 3 years at my church I can see why as many of my Windows devices start to have larger amounts of support requests generated at those points in time. Having scheduled retirements also assists with budget forecasts.
Obviously individual people will push hardware far beyond that that, and get away with it especially in small situations. I am not exactly interested in those edge cases.
How do I handle multi-year budget plans for replacements of Apple devices? What is the general rule of thumb that Apple devices should be replaced after X number of years? For people with 30-40+ Apple laptops, desktops etc, how do you plan for scheduled replacements?