I am guessing you are talking about protecting apples from codling moth, right?
There are several modes of action for kaolin. One of them is that it triggers an excessive grooming response from the pest and distracts it. Theoretically, it would seem that as long as there is kaolin on the surface of the fruit or the tree, then it should have some effect because I don't think kaolin would have a break-down "half-life" (like Spinosad) or a breakdown due to ultraviolet light. Kaolin, is after all, only an inert clay particle.
Having said that, I know in Western Oregon you would have to spray kaolin on a regular basis if you wanted crop protection.
Eastern Oregon? I would imagine you do not need to spray so often....but the truth is I don't know the answer and would like to know how often you need to spray. Would you be willing to find out and report back to us? Here's how I would go about finding out. I would call the Umatilla affiliate for the Oregon State Extension service and ask who is the Extension agent who works with sustainable agriculture and with organic growers of that area and see if you can talk to them. There is a phone number on this link: http://extension.oregonstate.edu/umatilla/
There is an excellent chance that they would refer you to a master gardener......Sometimes master gardeners are good at hunting down the answer and sometimes you simply wish they would connect you with a professional extension agent......
If you can't locate anyone with the Oregon State extension service you might try emailing Todd Murray who works in Hood River, Washington nearby and see if he would be willing to field the question. I met him in Seattle and while he isn't working in this field right now, he might still field the question or at least know someone who would.
Todd Murray email@example.com