This was addressed earlier by Chrom. The thing is that Russian fighters were built for a total war philosophy. They would be used until the nominal engine life expired. Then engines would be replaced. Western aircraft would use their engines until the engines began having problems, and then repair, and only eventually replace the engines. The Soviet approach was tailored to an all out war where the biggest threat wasn't maintenance issues, it was enemy planes and SAMs. The Western approach was more of a peacetime approach. Since then Russian philosophy on maintenance has changed significantly. Newer Fulcrums and Flankers offer significant improvements in engine life. Whether comparable to similar western aircraft or not is debatable, especially since conditions in which they are used are usually notably worse (in terms of spare parts etc.).
EDIT: Chrom I hope I correctly rephrased what you were saying.
Somewhat close, but i should add few points:
1. Engines were replaced, true. But that doesnt mean they were scrapped. They were overhauled in big depots. Only after several overhauls they were out of service.
Also, this philosophy was not aimed solely at wartime. Even in peace time there was considerable fund saving by reducing needs in good prepared airfields, high skilled technicians, and well equipped field depots.
In turn, this also somewhat increased strategical mobility of such forces due to above mentioned advantages.
2. Western technicians of course didnt waited till problems. This is far too dangerous. They, with various instruments, trying to determine reliability and wear of engine parts. It works well most of the times, just not always.
Also, some parts just get replaced after set amount of time. But again, many hard-to-reach parts require high skill, high working discipline and good equipment to replace - something what Soviet engineers tried to avoid as much as possible.
In Soviet conception most of these parts should be only replaced in big industrial depots.
