class MyViewController : UIViewController
{
    public MyViewController(IntPtr handle)
        : base(handle)
    {
    }

    private Timer timer;
    private bool timerEventBinded;

    public override void ViewWillAppear(bool animated)
    {
         base.ViewWillAppear(animated);
         if (timer == null)
         {
            timer = new Timer();
            timer.Enabled = true;
            timer.Interval = 60000;
         }

         if (!timerEventBinded)
         {
            timer.Elapsed += OnTimedEvent;
            timerEventBinded = true;
         }

         timer.Start();
    }

    public override void ViewWillDisappear(bool animated)
    {
        if (timer != null)
        {
           timer.Stop();
           if (timerEventBinded)
           {
              timer.Elapsed -= OnTimedEvent;
              timerEventBinded = false;
           }
        }

        base.ViewWillDisappear(animated);
    }

    private void OnTimedEvent(Object src, ElapsedEventArgs e)
    {
        //do your stuff
    }
}