Last night was quite a mark of efficiency. We cleared all 15 bosses in 25 man Naxxramas in 4 hours. Two raid resets ago we cleared three wings and reached Grobbulus in a night. One raid reset ago we left Frostwyrm standing after 4 hours and now nothing is left standing in that place.
4 hours is 240 minutes and 15 bosses means 240/15 = 16 minutes per boss including clearing trash and loot distribution.