how long does it take to get completely comfortable with flying an rc electric airplane?

It depends on the person. Obviously if you have been flying remote control for a long time then you will be able to do a lot more things and feel a lot more comfortable with your planes.

You will never be able to feel completely comfortable because every time you take your plane off the ground you will have that rush of seeing it fly and knowing that you are the one controlling it.

It could take you anywhere from 3 months to 1 year even to get comfortable with your planes. It also helps to fly the same bunch of planes that you've known well for a long time, that will help you a lot in feeling comfortable.

It also will help to put in some hours on the simulator. The new ones are amazing and would help you a lot in learning the basics of how to fly if you are a beginner.

Hope I could help and have a great time flying!!!!
References :
My dad and myself are very much into RC electric airplanes.
I've been flying for 3 years and feel extremely comfortable with my planes.