So, two things -
First, lets forget about physical fights for a moment. How about combining social and mental - seems oddly appropriate in some circumstances to have a social conflict during a mental conflict - say, arguing over a chess game? A mental conflict is not the same as a social conflict, but social manoeuvres seem oddly appropriate for a mental conflict.
Also, I don't think the rules explicitly state how long a round is. I always considered it arbitrary to the conflict. If you're having a footrace, a round might be a different chunk of time than a boxing match. I always considered the length of time a round took to be up to the narrator to decide what's appropriate. You do have to keep it the same for any given conflict - can't say one round is 6 seconds and the next is 10 minutes, but you can say a round in a wrestling match is different from a knife fight. The point is I wouldn't discount social attacks based exclusively on the length of time a physical fight takes as it's all relative. That said, do use reason when a player suggests an attack that would take much longer than an exchange to pull off.