After 6", I quit. Actually it depends on the ring on the schonstedt. I think I would be claustrophobic on that one.
I was told if you didn't dig down at least 12" you didn't really look for it.
There's always gut instinct, too. If I really thought it would/should/could be there, I kept digging. Can't count enough on my available digits how many times I did eventually find a monument at 4 feet or deeper.
That said, the depth in the video might be just a tad excessive. 😉
Those working for the DOT have been known to bring in backhoes and dig in a fashion no private surveyor would be allowed to do without huge compensatory damages being assured. Set up in the middle of the highway and dig down to the monument that was in place below a fill area at ridiculous depths, for example. There is a section corner I would love to have excavated but it will never happen. It is supposed to be something like 26 feet below fill where a side road ramps up to highway level. I would bet it is still there, quietly waiting for someone to expose it to fresh air.
My deepest handdug recovery was a bit over six feet.
My favorite was the inverted white plastic bucket down 3 feet and a marker message saying, "Keep going. You're almost there." Now that was helpful.