ButtonExtention.cs

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.Events;
using UnityEngine.EventSystems;
using System.Collections;

public class ButtonExtention : Button
{
    public UnityEvent onLongPress = new UnityEvent();
    public float longPressIntervalSeconds = 1.0f;

    private float pressingSeconds    = 0.0f;
    private bool isEnabledLongPress  = true;
    private bool isPressing          = false;

    void Update()
    {
        if (isPressing && isEnabledLongPress)
        {
            pressingSeconds += Time.deltaTime;
            if (pressingSeconds >= longPressIntervalSeconds)
            {
                onLongPress.Invoke();
                isEnabledLongPress = false;
            }
        }
    }

    public override void OnPointerDown(UnityEngine.EventSystems.PointerEventData eventData)
    {
        base.OnPointerDown(eventData);
        isPressing = true;
    }

    public override void OnPointerUp (UnityEngine.EventSystems.PointerEventData eventData)
    {
        base.OnPointerUp(eventData);
        pressingSeconds = 0.0f;
        isEnabledLongPress = true;
        isPressing = false;
    }
}