I've been riding seriously now for 15 years. For the first 7 years I had one bicycle which I only just barely maintained (oiled, got the spokes fixed, changed the tires, changed the cables). In the last 8 years, my collection has grown to 6 bicycles, and there were two others which I sold or traded in. So ... I guess you could say I've been upgrading, or at least changing/adding pretty regularly.

My newest bicycle, and the one I ride the most, is just over two years old. While I haven't bought a bicycle in the last couple years, I have done quite a few upgrades to that one to customize him even more and taylor him to what I want.

So I'd say that especially once you know what type of cycling you like to do, upgrades of some sort or another (new bicycles, new parts for the bicycle, etc.) are a normal part of a cyclist's life.