I use a triggered movieclip. When you press start or whatever starts the countdown, the bar movieclip tweens for 10 seconds and on that frame (240 if you're using 24 FPS) add a gotoAndStop, play, or gotoAndPlay.
except replace 15 with your framerate (fps). I don't know a command that returns the current framerate, so I have this which you can alter as you alter your framerate.
This way it will be flexible, as you can manually change the framerate var if you want to change the game's framerate. This is also more reliable than using a timer like, say, the PC's core millisecond timer. The reason is that, if your game slows down (fps drop), this timer will slow down with it. Otherwise, it could end up making it harder for players with slower computers, because they can do less in the allotted time.
So, yeah, there yah go. Each frame, one is subtracted, until the 150th frame where it will cut to gameover.