User blog comment:IamJakuhoRaikoben/Chapter 464: Natsu vs Zeref, Review./@comment-153.91.253.204-20151130200254/@comment-1776824-20151201191745

Lol, no. Natsu started using Igneel's power to fight Zeref without entering FDK Mode. FDK Mode is just a burst where he lets out all that power at once. Natsu's Magic Power has been enhanced with Igneel's own. He can use it for bits and pieces here and there, but ultimately it's finite and when it runs out, it's gone forever. All his feats are based on accumulated power of a true dragon. His power is reset, he is weaker, and he needs a power-up again. Hiro made all his stupid bullshit legit. Natsu has been fighting with the strength of a Dragon King.

And if Natsu says he spent 10 months trying to master something, when you try to master something, that's all you do. He spent almost a full year on Igneel's power alone.