Quick answer
A 12 mm drill with a 118° point has a tip about 3.61 mm long. To get 15 mm of full-diameter depth, the tip must reach about 18.61 mm. This estimate assumes an ideal conical point.
Formula and assumptions
Point length = (diameter ÷ 2) ÷ tan(included angle ÷ 2). Add it to the full-diameter depth. Applies to an ideal conical point.
Worked example
A 10 mm, 118° drill has a point about 3.00 mm long. To reach 15 mm of full diameter, the tip travels about 18.00 mm.