Hi speedster, I would agree with Geordie Travels point that it most often comes down to personal circumstances. For some people a replacement is necessary due to a change in circumstances, but for others it's because they want the newest in technology (lighter frames etc).
I've had my current wheelchair for around 6 years, and although I put it through it's paces on a regular basis, it's still in relatively good shape. I have had the upholstery and castors replaced, but as my physical needs haven't changed it still works well for me.
Hope this has been helpful and best of luck.