I was timing it today, it's 5 and a half minutes in 8 man, but it's worth noting that 16 man has to use 9 DPS (Not 10) for about 2.5x the amount of normal 8 man hp.
Today we managed to just barely beat the enrage with only 3 DPS up from 85% downwards, (Our merc DCed), so I'm thinking that 8 man just isn't tuned as hard for DPS as it could be. Either that or our tanks DPS carries us. Probably the latter
I thought the first kill had 10 dps, 3 tanks, and 3 healers. I did the math and 8 man and 16 man both required the same DPS per person (or at least extremely close) assuming the enrage timers were the same. If the enrage timer was different on 8 man and 16 man then it makes sense that it took them such a long time to beat it.
Damage comparison between 16 and 8 man mode:
3,265,395 HP / 10 = 326,539 damage per DPS
1,288,301 HP / 4 = 322,075 damage per DPS
Both of these assume the tanks aren't doing any damage. Tank damage should be slightly higher in 16man because they occasionally have two tanks on him at once don't they? Either way, it seems it would be balanced for them both to have the same enrage timer, which it sounds like they do now.