Keep user logged in, until they Logout

I have a login page with the following code:

protected void cmdLogin(object sender, System.EventArgs e)
{
if (ValidateUser(txtUserName.Value, txtUserPass.Value))
{
FormsAuthentication.RedirectFromLoginPage(txtUserName.Value,chkPersistCookie.Checked);
Response.Redirect("~/users/Default.aspx", Go to the complete details ...