If the grouphead gaskets start to wear, you will be experiencing pressure loss, which would result in easier pulling of the lever, and decline in quality of the pulled shots.
I think lifespan of those parts will greatly depend on how often the machine is used (1-2x per day, 10 times per day, etc.) and if you leave it on for extended periods or not.
FYI, november '09 I purchased a used EP from the late 60ies. The portafilter gasket was rock hard, mimicking a ring of bakelite (or the modernists around here: carbon fibre

), which indicated the machine had not been opened for maintenance in a long time either (needs to be screwed apart from the bottom).
I replaced the gasket just to test how it would perform, and up to now have not found the need to replace the inner seals. They are still taking the pressure as they should.
It is used daily for 2 shots within minutes after dinner, and more with visitors or during holidays / weekends.
If it ain't broken, don't fix it.