Well you nailed the question! I thought about offering numbers, but chose not to because they are crazy numbers. Since you asked... 75% of my riding is off road on rural rail trail and since I commute 34+ miles round trip, I know where to slow and stop, so I prepare for it and let the hub slow me to save battery power, not pad wear. I have 3 stop signs, 10 rail crossings each way and no traffic signals. Many days I have to brake for more wild animals than cars (very rural). I also rely heavily on the front brake, since most of your stopping power from speed is in the front brake.
So with the legalese done, I got more than 5000 miles in 10 months on my Avid Elixir metallic pads. That in my opinion is crazy! I went through the winter hoping the brakes would last, I checked monthly. Not only did they last when I went to change out my studded tires in early spring after 3000 miles, the pads had plenty of meat left.
I read accounts here and elsewhere of ebike disk brake pads lasting 1000-2000 miles, but pushing 6k! It must come down to materials and riding style. I hesitated with the numbers because I don't want to give the impression to others they can forget about their pads for 5k miles. I don't feel that's very common.
I replaced the pads with organic compound pads, they give a little better stopping power, but more important they never squeal wet, dry or dirty. I still had miles left in the old pads, but I got my money out of them and I wanted to replace the brake fluid and bleed the lines. I like the Kool Stop pads, I've used their pads and shoes on other bikes and knew they would be quality. If they last 3-4k, I'll be very happy.