The quick is a little bit like the horse hooves. You see the wall and the pad, so you trim as far as you can, trimming a little at a time. When you see the wall turn faint pink, stop. You can trim as far as the bottom of their pads and dont do anymore of it until the wall is grown back again. I 've done with my Nubians and they dont mind the trimmings every two months. Yes they do curl in a bit over the winter and when I check them on a warm late winter day, it is the time to do it. Also trim a hair off from the bulb of their pads since they do get a bit overgrown.
Hope this helps!