one season. I have a natural wood burning fireplace and after each season I re point my firebrick and then just spray the interior with cheap flat black spray. It makes it look clean and presentable for the OFF season, but it will deteriorate with the first fire. Now, a natural fire is much hotter than a fake log fire, so you might be able to find something that does work for you.
I have used rustoleum's high heat spray and it does not last either for me. But maybe for you.