We are dealing with the Time Stone here and also read this article regarding the timeline. Be aware that Strange was a genius and learned at an incredible rate. I don't think his training took that long. It fits in perfectly IMOWatched this again. Enjoyed it more the second time and really realized:
The Ancient One is a real hypocrite and Kaecilius and Mordo are right. The bill for their time fukkery at the end of the movie will come due. Isn't Darmammu going to be all obsessed with Dr. Strange now and want his revenge?
My only complaint is the timeline. It is all fukked up. It seems like he looked for a cure for his hands medically for years and then was at Karmar-Taj for at least a year to. That puts this story like a year or two or three ahead of Civil War.
When Is Doctor Strange Set in the MCU Continuity?