When do you trade queens

I assume it would be done in the midgame which I did not see a section for so I put it here, but how do you know when the right time is?

When you are ahead material.

That is the million dollar question.

The short answer is you attempt to initiate a trade when it is to your advantage to do so.  This might be when you are already up material and you want to trade down to a favorable endgame.  You trade when you can otherwise gaine some other advantage by doing so.

It's not always done in the middlegame... at times it's done early, while still in the opening.  Other times the endgame has been reached with queens still on the board.

There was a thread on this a while back, there are a few good ideas if you can wade through the spam:

http://www.chess.com/forum/view/general/when-to-exchange-queens2

Often at the beginning of the game (at least for me) each queen has a clear shot at each other once the capturing of the "sacrifice" pawns is complete. When I get the chance I usually capture the other queen, simply because it removes the option of a castle.

I usually trade if the opponents Queen is in better positioning and a nuissance to most of my other pieces.

When you get a winning pawn endgame: