NSTimer *myTimer = [NSTimer scheduledTimerWithTimeInterval:1 
    target:self 
    selector:@selector(myMethod:) 
    userInfo:nil 
    repeats:YES];

 - (void) myMethod:(NSTimer *) aTimer
{
     if (!pause) {
       // do something
       // update your GUI
     }
}

//and at last 

[myTimer invalidate];