In general, the more shots fired, the more copper fouling buildup.
It depends on a lot of things, the smoothness/roughness of the bore being the most salient factor. Some guns will build up copper fouling very quickly, some seem to almost be immune to the problem.
And is there a way to prevent the issue?
There are a number of things that can be done to reduce or prevent copper fouling.
Lapping the bore makes the bore much smoother and that can cause a significant reduction in the tendency of a gun to copper foul.
Some people say that using a bore break-in procedure will perform a similar function to lapping the bore, although it's probably not going to be as effective as lapping would be.
Simply cleaning the bore thoroughly (including the complete removal of all copper fouling) after every trip to the range will present a clean bore to the next few bullets that travel down the bore. That can result in the bore being smoothed out over time. Basically the same kind of effect that a barrel break-in procedure is supposed to perform.
There are new powders available that have properties that discourage copper fouling and may actually remove copper fouling over time. Those would only be available to handloaders.
What do you do once it happens, is the rifle basically starting to go downhill?
There are a number of ways to remove copper fouling, the simplest is probably using a good copper solvent when cleaning the bore. Another relatively simple method employs a bore paste that contains a very mild abrasive which polishes the copper away. There are also products which remove the copper fouling using a reverse electro-plating technique.
Copper fouling is not an indication that the rifle's useful life is coming to an end. It's just a fact of shooting like recoil or normal fouling buildup.