No matter what the content addition is it won't last months for people who are already at endgame. Of course people will go through it fast when it's one of the only things they haven't done yet. What people seem to be forgetting is what it means to new players. Dungeons are a great content addition, timer or not. Each addition gives new players another thing to look forward to doing. Over time that list just keeps getting longer and longer and they can't clear it faster than the devs can put it out. We can clear it that fast because it's all we have to do.

5 minutes is pretty insignificant but I don't really care either way. In Aion we had half hour lock out timers for some dungeons and it made little difference too. They also had 18 hour lockouts for certain dungeons and it was just more annoying than anything. Timers do limit how people want to play by telling them they can't go back into the dungeon yet. The people who have all the time in the world will still finish the content faster than the people with more real life obligations.