It's more a case of falling lucky when they offer a sample. So far I think I've had around 6 samples , two audio and three PowerCores, spread over 6-8 months
The samples are added randomly, on average a new one every 1-2 weeks, although I've seen this stretch to 3-4 weeks if they are trying to use up samples which have little uptake, such as screen protectors, after all not everyone has an iPhone 7 . PowerCores and audio products go fastest, often only a few are offered.
Best to check in sometimes morning and evening if your desperate to snag something. You can also email email@example.com to be put on a waiting list if you have not had the chance to review an item on the power user program.