Duelist of the Mind triggering "this turn"
Looking for clarification on [[Duelist of the Mind]] ability text that says it's power is equal to the number of cards drawn this turn. Since it says this turn and not each turn, my understanding is that on the turn it's played, it checks how many cards I've drawn. For example if it's it the first card I cast on my main phase, it will be power 1 since I have my normal draw. If I play a draw 2 spell, it will go up to 3 etc. Then, if play another draw 2 in my second main, it will go up to 5 and if I end my turn, it will remain at 5 power forever.
My opponent says it constantly checks and on his turn, if I don't draw any cards, the power becomes 0. He says it's something that is check every turn and I think it's locked in during the turn I initially played it.
Who's right?