I am sure you could make a mission with a limit of 25 minutes. Where after that time the game ends. However that means every game played will be the same or you need to make several maps for variety. If you really wanted to you could mod the game. I however am no expert at that. I do know though that the mission idea will work I am not so certain for a mod.
To Mankind
And the hope that the war against folly may someday be won, after all
Isaac Asimov