When does GMO (Genetically Modified Organism) become a non-GMO?

I have been reading for years how Monsanto got their foot in the door early on to provide GMO seeds to the masses even arranging it where all farmers had to buy their seeds from them. Now that we know that GMO food brings health problems, I’m seeing a lot of products marketed as non-GMO. If Monsanto is still in control of the seeds that are used in the farms that grow our food, at what point is it turned into a non-GMO??? If the original seed is a GMO, how many generations of reproduction does it take to return to a non-GMO state??? What rules are applied to a product at the grocery store in order for it to claim to be a non-GMO???

I hear a lot of homesteaders clam that they are growing non-GMO food. If the original seeds were GMO’d, how are they growing non-GMO food??? I know I have lots of questions but I’m searching for probably only a few simple answers that will cover most of them. What do you know about Genetically Modified foods???