Well you can be at the NM dragon around 4-5 minutes if people know what they are doing. You need 2 people at very precise locations to lock down each wave/act, and remember the golden rule: "Do NOT stop or run around try kill something far away from a spawn point"
The rest is up to whether you brought the right classes to take down the dragon, and if the dragon decide to troll the mele by flying away a lot. If people can not hold their own (you can only carry so much), then the game is obviously a lot longer, especially if one of those at the spawn points go down. It can be done in 6-15 minutes depending on groups class composition, promotions and equipment.