CHALLENGES - does anyone know for how long a challenge remains active? Does it stay up forever? or is the challenge expired after a certain time period?

It will stay until one of these things happen:
the players start making moves, turning it into an active game;
either one of the players deletes the game, using the "delete" button on their MyGames page;
Russ has one of his periodic clearouts of long-time inactive games. This seems to happen every couple of years.