I can vape 6mg on my RDAs. But 'can' is the operative word. That would really only be if someone else offered it to me. If using my own juice, its only 3mg. And, even with the 3mg, I still sometimes get a caffeine-like high (like what Baditude mentioned) off of the mass dose of nicotine... its controllable though if I slow down my vaping.
But its not overwhelmingly painful to vape 6mg. Anything higher truely hurts though.