How can I show admob more time

hi
finally i managed to create a code to display admob in my swf game with POZIRK extensions

my question is how can I show admob more time

I have this code for time it’s work but it’s show ads one time

var _admob:AdMob = new AdMob();
timer = new Timer(20000, 3);
timer.addEventListener(TimerEvent.TIMER, timerHandler);
NativeApplication.nativeApplication.addEventListener(KeyboardEvent.KEY_DOWN, deviceKeypress);
timer.start();