coruskate said:
Let me put the other view: a better lighter stiffer bike will be more fun to ride and reward your effort better, and you may find for that reason that you use it more and get fitter faster.
I have to agree with Coruskate there, although it very much depends on what kind of individual you are with regard to self-motivation, and how much spare cash you have.
I have ridden mountainbikes for years. One cost £350 when new, my latest would have been £4000 if I hadn't got it 2nd hand. And yes, it does make a massive difference IF YOU'RE RIDING HARD. If I wanted to ride the same trails the rest of the guys I ride with were doing, I needed a bike that could keep up. And there's no way my old bike could cope with what the new one will do. Mountain bikes are a bit different in that respect – with a road bike there's no real 'suspension' as such, so you're really talking about different degrees of lightness and stiffness.
I am one of those less-disciplined people who feels much more inclined to do something if its fun. I knew that if I didn't buy a bike that rode well and shifted well, it would lose its appeal. As I have had the fortunate experience of knowing how much difference decent shifters and wheels make (to say nothing of the frame), I decided it would be better to wait, save up more money, and then buy 2nd hand. Of course you do need to get the right size, and be able to tell if it's knackered.
Another way of deciding when to upgrade is to ride what you have (assuming it's rideable) and waiting until you can put your finger on exactly what it is about the bike that's holding you back: weight? shifting performance? frame flex? shopping basket on the front? Once you've got to the point when you know how much you would benefit from newer kit, it's probably time to do it. If you bought a bike with a great frame but the cheapest level kit, it might be worth doing it bit by bit - maybe a decent wheelset, maybe later some better shifters/mechs. Often though, it's better to take your time finding something that suits, and keep the old un as a winter hack (if you have the space and will actually ride in bad weather anyway).
Not sure that answers your question specifically, just my twopennorth.