ConfirmIt Set cookies using JScript.NET
The script will help you to set cookie for one year.
//surveyscripting.com
var myCookie : HttpCookie = new HttpCookie("year_expire");
var now : DateTime = DateTime.Now;
myCookie.Value = "true";
myCookie.Expires = now.AddDays(360);
myCookie.Path = "/";
HttpContext.Current.Response.Cookies.Add(myCookie);