For me, it takes about a year before I feel comfortable with a new area/job.
And still the learning curve continues, though not at such a steep rise!
Also, I agree: The first few weeks at a new position, after work I'm EXHAUSTED, literally, because I'm trying to absorb so much. It's not necessarily physically demanding, but emotionally draining.
Take a look at yourself in another month, and then in another three months . .
You'll be impressed with your progress, as you compare with how you were when you started.