Interesting , most of the high speed airrotor ( so called drill) works at a max speed of approx 400000 rpm( revolutions per minute ) Now in 2- 5 seconds at max speed and heavy force it has all the capability to drill quite deep into your tooth , but I don't wanna scare you ,,trust your dentist always as he or she has the power and the responsibility . And we don't need to drill so much if your tooth is fine,,,and if it requires deep drilling then we have to go more deep . In all the cases don't worry much coz we all are trained to handle that machine effortlessly.

Lol. It depends on dentist. Drill is used only to remover the infected part on the hard tissue I.e tooth . And no one tell you the depth as such ., It depend on speed and the light force and on the surface . But quite different question it is