Color me skeptical. Is a new $50 wheel going to be as good as what you've got now when you replace one (or a few) spokes?

As for when to replace the whole bike, that's largely a matter of personal preference. If you maintain your current bike well, you shouldn't run into the bow wave of components that all need to be replaced at once. Then it's a matter of saying, "I want a new bike and I'm ready to spend [fill in the price of the new bike you want] to get it."