Grid

using UnityEngine;

public class Grid : MonoBehaviour {

    public float time1 = 35f;
    public float time2 = 50f;
    public Vector2 gridSize = new Vector2(10, 10);
    public Vector2 distance = new Vector2(1f, 1f);
    public OnClick prefab;
    public Material mat;
    [HideInInspector]
    public OnClick randomGO;

    private float counter;
    private OnClick[,] instances;

    public void Start() {
        if(instances != null)
            foreach(var i in instances)
                Destroy(i.gameObject);

        instances = new OnClick[(int)gridSize.x, (int)gridSize.y];
        counter = 0f;
    }

    private void Update() {
        if(Time.time > counter) {
            var x = Random.Range(0, (int)gridSize.x);
            var y = Random.Range(0, (int)gridSize.y);

            while(instances[x, y])
                if(++x == (int)gridSize.x) {
                    x = 0;
                    if(++y == (int)gridSize.y)
                        y = 0;
                }

            instances[x, y] = Instantiate(prefab, transform.position, transform.rotation) as OnClick;
            instances[x, y].transform.parent = transform;
            instances[x, y].transform.localPosition += new Vector3(x * distance.x, 0f, y * distance.y);
            instances[x, y].name = string.Format("{0}-{1}", x, y);
            instances[x, y].grid = this;

            var hasSpace = false;

            for(x = 0; x < (int)gridSize.x; x++)
                for(y = 0; y < (int)gridSize.y; y++)
                    if(!instances[x, y]) {
                        hasSpace = true;
                        break;
                    }

            float time = Random.Range(time1, time2);
            counter = hasSpace ? (Time.time + 1f / time) : float.PositiveInfinity;

            if(!hasSpace) {
                x = Random.Range(0, (int)gridSize.x);
                y = Random.Range(0, (int)gridSize.y);

                randomGO = instances[x, y];
                randomGO.GetComponentInChildren<Renderer>().material = mat;

                Debug.LogFormat("Random GO is {0}", randomGO);
            }
        }
    }
}