try below code :-)
<asp:Literal Text='<%#Limit(Eval("feedback_details"),40)%>' id="Literal1" runat="server"/>
protected string Limit(object Desc, int length)
{
StringBuilder strDesc = new StringBuilder();
strDesc.Insert(0, Desc.ToString());
string result ="";
if (strDesc.Length > length)
{
result = strDesc.ToString().Substring(0, length) + "...";
}
else result =strDesc.ToString();
return result.Replace(Environment.NewLine, "<br />");
}