static void Main(string[] args)
        {
            var m1 = new int[]{ 1, 2, 3, 2, 3, 5, 5, 3, 7, 1, 2, 4, 8, 9, 11, 12, 1, 1, 1 };
            var m2 = new int[] { 1, 2, 3 };
            Console.Write(MaxLength(m2));
            Console.Read();
        }

        public static int MaxLength(int[] arr)
        {
            if (arr == null) return 0;
            if (arr.Length < 2) return arr.Length;

            var max = 0;
            var i = 0;
            var j = 1;
            var curLen = 1;

            while (j < arr.Length)
            {
                if (arr[i] <= arr[j])
                {
                    curLen++;
                    i++;
                }
                else
                {
                    if (curLen > max) max = curLen;
                    i = j;
                    curLen = 1;
                }
                j++;
            }

            return Math.Max(max, curLen);
        }