Well described Raul. The failure mode must have a time or usage based distribution. In Weibull analysis it has a beta value greater than 1. The larger the value, the more strongly related to aging the failure is. As in all proactive work we are aiming to reduce the risks of failure. We do that by reducing what happens when the failure occurs (consequence mitigation) or by reducing the probability it will happen. With age related failures, where preventive measures can be effective, they actually prevent most of the failures if they are performed at an early enough time.
I've seen quite a few instances where preventive replacements were used, expecting to improve performance of the assets but failing. When asked about task frequency, a number have studied their work order histories and determined MTBF, then used it as the task frequency. Big mistake! MTBF is the average age at which failure can be expected, i.e.: 50% will have failed by then, the other 50% will still be working. The task frequency needs to be quite a bit less than MTBF.
If MTBF is known then you probably have enough data to perform a Weibull analysis and can determine Beta. You then have a good idea what the probability curves look like. The area under the probability density function represents is the % of failures that have occurred up to that time. If you can tolerate say, 10% failing before reaching the PM interval, then choose that time as your interval.
If you do this, you will experience roughly 10% failing by the time you get to the interval you chose and 90% still running. You'll replace the 90% and restore or discard the items. It is expensive to do that and most of the items will appear to be just fine. Many trades will argue that you are throwing away good parts (and you are). The whole purpose is to reduce the risk. To make sure you aren't doing that needlessly, you need to be certain that you are dealing with an aging failure, and that it is worth the costs of discarding all those items when they are replaced. That's a simple economic calculation and it must consider the costs of downtime and any other secondary damage that might be associated with the failures if they are allowed to occur.