Well, several years ago the five top breeder families got together and formed a organization called Herper Inc. whose job it is to whack any breeders who authorized unsanctioned hets.
Just kidding. There is no enforcement and to make a slight correction, a lot of breeders, not just reputable ones, will offer a genetics guarantee on their animals but only a reputable one will stand behind it. If you browse the BOI on Fauna, you'll see some cases where some supposedly good guys, who have been around for many many years, failed to back the guarantee or offered, in my opinion, very little compared to the damage done.
Trust the breeder, not the guarantee itself, because it is worthless if the breeder doesn't stand behind it. There is no set number of clutches or eggs. It'll be until the breeder feels its enough, and even then, may want the snake back to cross himself if it's a high dollar snake.
To give you a ballpark, I think there was a recent case where a het was declared as disproved after 36 eggs? I can't remember if it was crossed with a visual or het. Personally, I would never accept a claim if a het was crossed with a het because you can't control the legitimacy of the other het... should only be visuals to disprove.