I have stop collars, but I really don't much care for them. Maybe they're just cheap junk, but mine just don't work well.

What I prefer, is to wrap some electrical tape around the bit, to mark the intended depth.

For something super critical, a length of tubing can be slid over the bit, leaving exactly enough bit exposed to drill the required depth. The tubing will act as a positive drill stop, even if the drill is pulling itself into the material.