I've had a couple. They need to get into the roots and pull out all the stuff from inside there. One of them took longer than the other. They had to do X-rays in the middle to see if they got all the stuff out. Once done, they will have to fill back in the tooth. See if you can get a white (porcelain) filling if you want it to look like regular teeth.
They numb you up so that shot kind of hurts although nowadays they will put a topical numbing agent on your gums to help with the shot part.
I don't recall having terrible pain but there was some discomfort afterwards. I tend to chew on the other side for a few days after any dental procedure (fillings, crowns, root canals) so I couldn't say exactly when it come better.
I have TMJ issues so my jaw was sore from keeping it open for so long. Ibuprofin or something similar will help.
They may optionally offer to put something in your mouth to help keep it open during the procedure. I usually decline because I have a strong gag reflex.
On one of mine they ended up putting this thing in my mouth around the tooth they were working on to help keep my tongue out of the way. It was both good and annoying. Annoying because there is this extra thing in your mouth. Good because I felt more comfortable about being able to swallow whenever I wanted because I knew my tongue wouldn't be bothering the work area.
I had one take long enough that they needed to give me another novacaine shot midway through the procedure. The other time that didn't happen.
I recommend hitting the restroom before you start. I tense up during the procedure even though it doesn't hurt with the novacaine so by the end, I really have to pee.
Good Luck.