计算for循环的大概运行时间

我的C＃Windows窗体应用程序中有一段代码如下：

List<string> RESULT_LIST = new List<string>();
int[] arr = My_LIST.ToArray();

string s = "";

Stopwatch sw = new Stopwatch();
sw.Start();

for (int i = 0; i < arr.Length; i++)
{
  int counter = i;
  for (int j = 1; j <= arr.Length; j++)
  {
    counter++;
    if (counter == arr.Length)
    {
      counter = 0;
    }
    s += arr[counter].ToString();
    RESULT_LIST.Add(s);
  }
  s = "";
}
sw.Stop();
TimeSpan ts = sw.Elapsed;
string elapsedTime = String.Format("{0:00}", ts.TotalMilliseconds * 1000);
MessageBox.Show(elapsedTime);

我使用此代码来获取“我的列表”中数字的任意组合。我对My_LIST的行为就像递归的那样。下图非常清楚地说明了我的目的：

我需要做的就是：

编写一个公式来计算这两个嵌套的for循环的大概运行时间，以猜测任意长度的运行时间，并帮助用户知道他/她必须等待的大概时间。

I have used a C# Stopwatch like this: Stopwatch sw = new Stopwatch(); to show the run time and below are the results(Note that in order to reduce the chance of error I've repeated the calculation three times for each length and the numbers show the time in nano seconds for the first, second and third attempt respectively.):

1. arr.Length = 400; 127838 - 107251 - 100898
2. arr.Length = 800; 751282 - 750574 - 739869
3. arr.Length = 1200; 2320517 - 2136107 - 2146099
4. arr.Length = 2000; 8502631 - 7554743 - 7635173

Note that there are only one-digit numbers in My_LIST to make the time of adding numbers to the list approximately equal.

How can I find out the relation between arr.Length and run time?