you don't need to use regular expressions, you can just use String's TrimEnd method, but if you insist,
  string xx = "T:,0,e:,n,l:,1500,,";
  xx = xx.TrimEnd(',');
  Console.WriteLine(xx);    xx = "T:,0,e:,n,l:,1500,,";
   xx = System.Text.RegularExpressions.Regex.Replace(xx,",+$","");
    Console.WriteLine(xx);