1 = Depends on the instance, the group, and the skill :0 On average, I've done Utguarde in an hour, Nexus in under an hour, and Azjol-Nerub took like 3... but we failed and didnt know what we were doing. I know fully expect it to be doable within 45 minutes.
2 = 77 scrub geared feral druid, 71 BT & Sunwell geared healer (me), 72 mage, 71 & 70 lock - all dps variued between t5 & badge to sunwell epics.
3 = Utguarde is Ramparts, Nexus was like Botanica (plants), combined with Arcatraz / tk (Lots of shiny glyphy things), but filled with dragonkin, and Azjol-Nerub was short, sweet, awesome looking (all spider & spider webby) - but the first mini boss required some figuring out (1st attempt for the whole group).
Have some vids of utguarde & Azjol to put up later... large fraps files don't convert nice while playing