If you can make a dent in the pitch after 10 to 15 seconds, it is probably soft enough. If the pitch is too hard, add some mineral oil to it. Use around a tablespoon per pint of pitch, then test again. Repeat if necessary. If it is too soft, just keep cooking it until it passes the thumbnail test.
Some folks add some turpentine also.
Don't add too much though, Turps softens things up more than mineral oil. The finished lap will harden up faster also if turpentine is used. I also don't like the idea of boiling pitch. Remember that pitch is a flammable substance 3. Make sure there are no leaks running down the sides.