Your slave cylinder is within a sealed system, so it will generally last a very long time, and may never have to be replaced. The key word here is, of course “may.” If you’re in the habit of buying newer vehicles, and trading in on a regular bases, the chances of you ever having to deal with slave cylinder failure are low.
That said, there’s really no defined time period for clutch slave cylinder replacement – it’s not a part of your regular maintenance. Chances are that if you own your vehicle for long enough, or if you’ve bought an older vehicle, it’s not unreasonable to think that at some point, the slave cylinder will begin to leak. A slight leak might not be the end of the world – you’ll notice that your clutch isn’t working as well as it once did, and at this point you may be considering replacing the vehicle in any case.
Keep in mind, though, that if you plan on continuing to drive the car, you are going to have to have the problem dealt with. You’ll need to have leaks diagnosed to find out if the problem is in the slave cylinder itself, or in the clutch master cylinder. If you wait, and the leak gets worse, you could end up with complete slave cylinder failure, no clutch, and a vehicle that isn’t drivable. So, if you suspect that your slave cylinder is leaking, you should enlist the services of a professional mechanic who can replace the slave cylinder for you.