Well, the important point in a coin swap is reaching the coin holders to inform them about the swap. And, for a long list of reasons this is not an easy task. Never missed a coin swap in your life?
So, for a successful swap, my two cents, it is important to make sure the news has the proper visibility, or many people will be left out.
If the coin to be swapped is on the exchanges, I would say that should be done pretty quickly, but if the coin to be swapped is dead and no longer traded, leaving the swap open for a longer period shouldn't create big issues.
I would say 3 to 6 months, but I'm curious to know other people opinions as well...