I try for every 3 years, because after that the warranty is out, I start to get nervous... Plus every 3 years, there's been enough advances in computing that I can justify the new machine🙂
The tax folks allow a business deduction on equipment & machinery, depreciating its value over time. Computers (like desktop & notebook) are a three-year schedule.
The last laptop I bought before buying my macbook for christmas was a used dell laptop back in the fall of 2004. I think it was 1-2 years old when I got it. A latitude C400 with a PIII 866MHZ lol. That sucker has had the battery changed and the screen change one time each since I've owned it and is still kicking strong today. I have since given it to my brother to use. This was probably the most reliable computer I have ever used and it has been abused like no other.
If my macbook is anywhere near as reliable as that thing, I should easily be able to hold on to this one for at least 4 years. Hopefully more.