Metodo onStartCommand (androidos-lab.it)

 @Override
    public int onStartCommand(Intent intent, int flags, int startId) {

        final Handler handler = new Handler(){
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                Toast.makeText(getApplicationContext(), "Passati 5 secondi", Toast.LENGTH_SHORT).show();
            }
        };

        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    try {
                        handler.sendEmptyMessage(0);
                        Thread.sleep(5000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        t.start();

        return 0;
    }