On my Priest I ran Durn twice before I realised the reason I wasn't getting any quest advancement was because I didn't pick up the quest after doing the initial tour of the caverns quest. Anyway, after doing that initial tour quest and two Durn runs I have 2246/3000 rep (Neutral).

So seeing that the tour quest gave me 250 rep, that means that I got 1996 rep just from 2 runs of mobs & bosses. Averaging 998 rep per run. Apparently a full clear gets you 1200 rep and you can grind Durn up to Exalted.