Hard to say but at that mileage I'd invest in doing the water pump, timing belt and oil pump seal all in one shot. Three problem areas which can be addressed before you're stranded somewhere.
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No doubt, but I would add that heat is a big factor too. When I replaced the valve stem seals on my 3VZ, the old exhaust valve seals crumbled like dried up peanut butter. The intake valve seals were in much better shape, but still very hard compared to the new ones.It wasn't the mileage that made them hard, it was age.
Yeah, those valve stem seals probably have the roughest life possible for an elastomeric seal. Here's a pic of the new ones when I was doing the HG job on my 3VZ. These sit on top of the valve guides.Agreed on the temperature factor...the oil seals are probably subjected to a lot more heat. BMR, I didn't realize the valves had seals other than the metal guides they operate in. There are some non-metallic seals in the valve guides now?