Pipe tap depth question

All of this talk about taps and tapping speed reminded me of a
problem i always have when trying to use a pipe tap.

How do you know how far to thread a pipe tap into the workpiece for
proper pipe fit. I usally tap a few turns then take the tap out and
try the tapped hole with a pipe fitting, then tap another turn or 2
and test again.

This process is slow and sometimes i tap to far. There must be a
better way....

WallyK