EDIT Oh and its a ROUND that is 6 seconds not a TURN.
I think in most (almost all) situations a turn might reasonably be considered to map onto a time span of six seconds or whatever the length of a round might be in your game. However, there are instances where a creature can have more than one turn in a round and, therefore, a turn couldn't be six seconds.
We don't often (ever?) consider how much time a player's turn actually takes. We just know that if they can do it in a turn it can't be more than six seconds. I know I don't care how long the actions a players takes actually last if the player can perform them in one turn. D&D maps simultaneous actions onto a sequential resolution system so things are inherently askew from the start.