Does a game stay on sale for a certain amount of time, or does it stay on the market as long as it is "selling well?" If the latter, what constitutes selling well?
Well, I think it is obvious that games with better ratings and more hype etc stay longer on the market.
The only way to help it stay longer on sale is to have the luck having the game being promoted on tv or doing some commercial before the game's release and while it's on the market. Still the biggest factor for a good and long sale are the amazing genre combinations.
Take a look at them here: Amazing Combos and more
I hope the answer is not too late :)