You really could use the regular expression /((s).*?){n}/ to search for n-th occurrence of substring s.

In C# it might look like this:

public static class StringExtender
{
    public static int NthIndexOf(this string target, string value, int n)
    {
        Match m = Regex.Match(target, "((" + Regex.Escape(value) + ").*?){" + n + "}");

        if (m.Success)
            return m.Groups[2].Captures[n - 1].Index;
        else
            return -1;
    }
}

Note: I have added Regex.Escape to original solution to allow searching characters which have special meaning to regex engine.