For how long can you rely on weathersealing?

Since getting my E-M5 and 12-50mm lens last year I have made use of their weathersealed properties on a number of occasions, but have found the maximum aperture of the lens a bit limiting given that the lighting conditions are generally quite dim. I have been wondering about some alternatives which in the native mount are quite expensive, and have found a former studio selling a whole bunch of used Olympus 14-54mm f2.8-3.5 MkI lenses for only $AU129 each. I'm aware of the AF speed limitations of these lenses on my E-M5 which is still a big consideration, but my greatest doubt is how well the seals age over time given that the lenses might be up to 10 years old. My sole interest in them is as a weather-sealed zoom since I have a good weather option so if I can't rely on the seals I won't worry about it.

Thanks.