Depends on what kinda team you're running I guess. For my shammy team (5xshammy) I ran a total of 8 badges pr day (10 if I hit the daily heroic) in about 2 hours (sometimes more if I had accidental wipes).

Started in Ramparts, did the first 2 bosses. Went to Steamvault and cleared up to first boss while earth elementals were on cooldown. Did the bosses here (about 20 minutes afk time in here due to earth elemental cooldowns). Then lastly went to Slave Pens and cleared it. Pretty fast & rewarding, and doable as freshly dinged 70's almost. Very easy and rewarding for a non-pve setup. Expect alot of repair bills until you get the hang of it though.

As a pve team, I would still say Slave Pens is maybe the easiest (although harder for a pve team than a shammy team ie). Sethekk Halls could be good if you're steady on the CC, but not very rewarding since it's only 2 badges. This instance was pretty hard and unrewarding for my shammy team, but I don't think my pve team would have much trouble (still untested). Steamvault on the other hand is pretty hard for a pve team. The first boss is the hardest challenge in here I feel. Doable of course, but kinda unforgiving, so you really need to be on top of things here. Mechanar is also fairly easy, but skipping the 2nd boss, it's not very rewarding as far as badges/hour is concerned.

If you can do them, Black Morass, Durnholde and Botanica are very rewarding. I'd say about 4 hours of work for 11 badges.