[abcd_f]

For the lazy - https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole

Edit: from the Russian version of the page -

A single drill bit lasted about 4 hours, which was enough to drill between 7 and 10 meters. In comparison, raising and lowering the drilling column took up to 18 hours.

It also mentions that due to the core samples being saturated with hydrogen, they turned to a fine dust unless they were raised very slowly to allow the gas to dissipate.