There is a lot of variability in how often you have to change seals in a cylinder. You have to ask yourself, how often are you using the cylinder? That’s the first question. How fast is it moving? High velocity seals? How contamination free is your fluid?
Ideally, you should never have to change your seals. If your fluid is clean, if you’re keeping it cool. In severe applications, really, the seals should last forever. If you look at, say, a modern automobile engine that can last 300,000 or 400,000 miles with very little changeover of internal components, why can’t your hydraulic system be the same? You just have got to take care of it. If you take care of your fluid, your fluid will take care of your seals.