Based on what Lanefan said and the Legolas example, I think a turn isn't really so much about how long any activity takes, but about how often everyone engaged in combat has an opportunity to evaluate the current situation and make a descision how to change his activities accordingly.
And I think 6 to 10 seconds is not a bad frame of reference there.
If I chose to take an attack action, my character keeps attacking his target and he gets one opportunity to roll to see if he is wearing his enemy down.
If another character shouts for help, I wait for my next turn and then I can decide if my character keeps attacking his enemy or turns around to aid his ally.
And if I keep fighting with my enemy but my ally is still in danger and nobody else came to his help either, I again can decide if I turn around or stay on my next turn.
And I would not want to have to wait for more than 10 seconds to get my next opportunity to react to changes in the battle.
If I think the enemy might be dead within the next 6 seconds and my ally can wait 6 more seconds, I would first kill the enemy before comming to the rescue.
But if I have to consider if I will have killed the enemy in the next minute and if my ally can hold out a full minute without help, making that descision would feel very strange to me.
How often I can decide to change my current activity also means for how long I commit to a single activity. And I never have been in a sword fight, but I don't think a fight to the death without rules between two armed people would ever last more than a minute or two. A minute is a really long amount of time.
If a rogue needs to minutes to open a lock while his allies are supposed to keep his back safe from a small army that comes down the corridor, then it's most probably not going to happen that he makes it in time. It's too long, it can't be done. Saying that everyone in the fight is just taking 10% of the normal damage to indulge the rogue seems to be completely the wrong solution. You can't just ask the monsters and NPCs "Would you mind going a bit less agressive if we do the same? We'd rather roll for damage only two times than twenty times."