private System.Windows.Forms.Timer timer5mins; 

    private void FiveMin(){
        timer5mins= new System.Windows.Forms.Timer();
        timer5mins.Tick += new EventHandler(timer5mins_Tick);
        timer5mins.Interval = 1000; // 1 second
        timer5mins.Start(); 
    }

    private void timer5mins_Tick(object sender, EventArgs e)
    {
          //Call Webservice Method
         //You can either ReCALL FiveMin Function Again to never stop
        //Or
       // Use timer5mins.Stop(); 
    }