Setting a time limit on game?

1

I am making a simple shooting game and have a couple questions. I have all ready done some tutorials and have a basic game done.

  1. How do I set a time limit / count down timer?

  2. I have done the tut on sitepoint.com, and the problem is each targets show up once, you make your way through the game by taking them out, then its over. I want to know how to make target show up for a couple seconds then be gone. For example, a guy will peek out from behind a crate stand there for 2 seconds, the duck back down.

So basically I want to do a game that last for about a minute. There are four places that guys can jump out from. They peek out for about 2 or 3 seconds then duck back behind. While they are visible, you can shoot them by using the mouse.

I know that this game has been made a million times, but I am new to making flash games and fiqure it is a good place to start.

Thank You