I think it depends on the professional and how you define Professional. It's important to note that everyone goes at their own optimal pace.
And while its good to see how the more experienced pros do it, it's the work ethic and mindset you want to start with emulating, not their panel count.
Because something like efficient quality drawing is something that comes with years of diligent practice, what I like to call training cause that is practice with a goal in mind.
My personal definition of Professional is someone who shows up each day and does their chosen work consistently, whether or not you get paid for it. So long as you show up and create and finish something, I consider you a professional. Note this is more about mindset.
"Well done is better than well said" -quote from I want to say Ben Franklin but I might be wrong.
In terms of personal experience. Back in August, I learnt that I was chosen for the Incubator Program Second Class. This meant I could work full time of my upcoming comic Swaha.
I've spent like 15 years now learning to draw fast and accurate. I personally like to focus on strong line work.
Nowadays I can get a solid 5-7 hours of 'arting' in a day (this includes not only production--the drawing part--but reviewing and editing the comic to ensure the story is being told clearly), five days a week.
It does demand a good chunk of time and I'm incredibly fortunate to be in the Incubator and in a situation where I am compensated for my work so I don't have to keep freelancing on the side.
Currently my process is more or less of the following (the script is complete):
5 main "stages" of the panels:
Thumbnails (40 -65 panels can get done in a day)
"Pencils" or Clean Sketches + Dialogue bubbles (30-50 panels)
"Inking" or Line work (20-40 panels)
Flat colors (20-40 panels)
Rendering (20-40 panels)
I always find it takes longer than anticipated to do the latter stages and you also have to consider the time needed to fix any mistakes or go back and make edits.
That said, each Episode is 55 to 65 panels long, and right now it appears I can get that done in two weeks (ten days cause I rest on weekends usually) once I really figure out my workflow for maximum efficiency. That is always a work in progress.
I'm trying to get more sleep but I manage to average 7-8 hours a night. After five on Friday I stop working so I can have time with friends.
Resting is just as important as working. In life, for me, having healthy fulfilling relationships and time for myself is what replenishes the creative wellspring from which I draw the energy I need to create good stories. Maybe that means I can't finish an episode every two weeks in the short term right now.
But it does mean I will have a healthy lifelong career in webcomics without hopefully any injuries or negative artist stereotypes.