There is nothing 'normal' about the wait time for anything custom and I wish people would stop thinking that there is. Production is one thing (and very predictable) but custom is a totally different animal.
The time it takes is the time it takes.
The last thing you want to do is to prod an artist to go faster. Neither of you will like the result. At that point, YOU will be responsible for your money and his time having been wasted.
If you want quick, buy production. If you want custom, acquire the patience to be worthy of the artist's efforts.
Sistine Chapel = "It will be done when it's done".