There is a lot of variability in how often you should replace your cylinder seals. First, you have to ask yourself some questions.
How often are you using the cylinder should be the first question. How fast is the cylinder moving, and do you have high velocity seals installed on it? What’s more, how contamination-free are you keeping your fluid?
Ideally, you should never have to change your seals. If your fluid is clean, and if you’re keeping it cool in severe applications, really, they should last forever. If you look at a modern automobile engine that can last 300,000 or 400,000 miles with very little change of internal components, why cannot your hydraulic system? You’ve just got to take care of it. If you take care of your fluid, your fluid will take care of your seals.