@Füty *****
There is talk about extending the amnesty to sometime late September, however that has not been formally enacted into law. Because of this, the mods and admins of this group currently assumes the amnesty will end on July 31, 2020, and our advice reflect that.
When you cancel an extension of stay, you have up to 11:59PM *THE SAME DAY* to leave Thailand, assuming there is no amnesty. If you want to stay longer, you can apply for another extension of stay, which will be denied, and you will have 7 additional days to leave (assuming, of course, there is no amnesty).