DateTime dt = DateTime.Now;
for (.......)
{
}
TimeSpan ts = DateTime.Now - dt;
textBox1.Text = ts.TotalMilliseconds.ToString();

或者(根据MSDN)如果你需要更好的分辨率

Stopwatch stopWatch = new Stopwatch();
stopWatch.Start();
for (.......)
{
}
stopWatch.Stop();
textBox1.Text = stopWatch.ElapsedMilliseconds.ToString();