Estimates will vary greatly based on how much hair you have on your face going into it. I had incredibly thick, dense, and persistent facial hair. Permanent 5 o'clock shadow even immediately after shaving [imagine
this but darker/thicker and covering all of the cheeks even up near the eyes]. I had laser treatments every 7-8 weeks for 5 years. It took me 3 years to get "full clearance", and even after that, I had some regrowth every time I went in for a session. In hindsight, I probably stuck with laser 2 years too long. I could have transitioned to electrolysis after the first 3 years.
Anyways, my understanding is that laser is not permanent. Since I stopped getting laser treatments, I have had significant regrowth, which my electrologist has tried valiantly [but not altogether successfully] to keep on top of. We're making steady progress though. What laser did do for me is make the hair thinner, and grow a lot more slowly. That, in and of itself, was actually a massive relief.
My electrologist estimated between 180 and 240 hours when I first went to see her based on what I had on my face at the time, and projected regrowth. She told me she gets plenty of "laser refugees" and she also works with many transgender clients. So I trusted her judgement. I've put in about 60 hours so far, and she's since adjusted the estimate to around 120 to 150 hours.
RE: scarring. I've read that the risk is higher with thermolysis or blend techniques [due to the higher speed, and temperature]. Although multi-needle galvanic is slower, it is also safer. As well, the galvanic technique has a higher success rate per follicle compared to the other two.
RE: pain management. I've found that the topical creams are only mildly helpful on the surface. As in, they help with numbing the probes entering the follicle, but not so much with the steady current build up. At the moment I'm just making do with two OTC ibuprofen + codeine tablets that I take right when we start. Makes the first couple of hours easy, and I just grit my teeth and push through the third. When I started, I was doing 4 hours straight, but the fourth left me really frazzled, so I dropped to 3 at time.
Anyways, if your lady works with trans clients, I'd say you're in safe hands. Don't be afraid to ask questions and make sure she explains what approach she recommends for you, and why. Good luck and hugs