I doubt there's a proper dimension for that because it's not a way that very many fields are set up. Anyway, the field linked below is a HS field that's turf with dirt cut outs. Using the known dimensions between the bases, you can figure that the cut outs go about 18' in either direction from the bases.
(must be a real pain to keep the dirt out of the turf and the rubber pellets out of the dirt)
edit -- actually I think i am misremembering that. It's been a couple years since I was there. Looks like only the mound is dirt. But something like 15' to 18' sounds good to me!