Juddz, It depends upon where the dent is, how hard it is for them to get behind it, and how long it takes them to "massage" the dent out. If the dent is in a leading edge like a crease around a fender lip, or a design crease or body site line, then it takes them a little longer to get it out. That is when you compare it to the broad flat expanse of the door.
Like PGA stated, it's never taken them more than 15 or 20 minutes to fix any of mine.
I've paid as little as $60 and as much as $135. Always been satisfied with the result. Sure beats having them "bondo'd" up, and repainted (which never seems to match on a metallic color).