To answer your question simply, it is common to take one 30mg hit only. Anything over 50mg is most probably an overdose. Try and get it in one hit, obviously that is hard with a vaporizer so that is why you take 3 hits.
I am not a fan of this method as "the sweet spot" is different for different people and certain sensitive people are at risk of overdosing. I have often seen the opinion on here and elsewhere that "all you need is a glass vapor genie or equivalent, plug 30-40mg of freebase and hit it three times and if that doesn't work SMALK MOAR!" when in reality, at least for me it has been ineffective at getting a stable level of dosage. Now I use an e-cig device that gives me a constant hit of 13.424252315234321243673845 mg per hit :d or dab it