if (this.ChkRememberme != null && this.ChkRememberme.Checked == true)
{
int timeout = rememberMe ? 525600 : 30; // Timeout in minutes, 525600 = 365 days.
var ticket = new FormsAuthenticationTicket(TxtUserName.Text, TxtPassword.Text);
string encrypted = FormsAuthentication.Encrypt(ticket);
var cookie = new HttpCookie(FormsAuthentication.FormsCookieName, encrypted);
cookie.Expires = System.DateTime.Now.AddMinutes(timeout);// Not my line
cookie.HttpOnly = true; // cookie not available in javascript.
Response.Cookies.Add(cookie);
}
Go to your web.config and find the authentication element. You can set the cookie expiration time (in minutes) there, like such:
<system.web>
<authentication mode="Forms">
<forms loginUrl="~/Account/Login"
name="myCookie" <!-- optional, if you want to rename it -->
timeout="2880" /> <!-- expires in 48 hours -->
</authentication>
</system.web>
Source: how to apply "Remember Me" in c#
Hope this helps
Happy Coding..!!