If there are trace amounts of 40K you would need about 5 half lives, so about 6 billion years. At that point the Sun will have gone into a red giant phase and swallowed up the Earth, so dating rocks would be the least of our concerns. In order to get rid of all the 40K you would probably need 10 half lives at a minimum, so 12 billion years. Needless to say, there should still be ample 40K available since it was produced in the star and supernova that gave rise to the heavier elements in our solar system.
However, there is a lack of isotopes with short half lives that aren’t the product of decay from longer lived isotopes.
https://www.noanswersingenesis.org.au/is_yec_heresy_jm.htm
Once you get into half lives in the range of a few million years you no longer see those isotopes naturally occurring on Earth. This is yet another clue for the ancient age of the Earth.