When I took a class, ages ago when I was very young -- it was what got me into comics and sequential art in general -- we were studying the book How to Draw Comics the Marvel Way. The original one, the good one. Our instructor, named Anthony Spallini I want to say (whatever happened to him? I hope he had a good life), told us that a panel might take upwards of an hour to complete. In those days, it really did usually take an hour.
I don't do color comics, so let me just say that before I get into my time figures, but nowadays we of course have great things like Manga Studio/Clip Studio Paint to make comic production much, much easier and more streamlined. These days, it takes a couple of hours at most for me to finish the visuals of a page, most of the time, unless it's an especially complicated page. There are so many brushes and tools and tones that can take care of something in less than 30 seconds what used to take the better part of an hour! And every day, I'm happier that I made the leap from traditional with digital finishing to fully digital.
Dialogue actually takes me a while usually, because I want it to sound organic and come from an organic place. I usually will do all the visuals of a chapter and then go back and write the dialogue all at once, so that it sounds natural and is consistent. But that's different to how some people work, and many enjoy having a script from the start. I just can't work like that if I'm doing something I want to have passion about, though.