To determine the sampling frequency, you need to ask yourself what you want to find out and how fast it needs to be found. Obviously, cost can also be a factor.
When deciding on how often to sample, you should take into account the equipment history and the likelihood of failure.
More frequent sampling gives better data trending and also picks up on possible failures as they’re happening. Bear in mind that failures can happen quickly and so a unit that’s suspected of having a problem should be sampled frequently, especially if it is a crucial machine.
If a sample is taken only yearly, the lab analyst will not have much data history to make a comparison to. In contrast, monthly sampling provides a 90% range of sample detection before unit failure.