How to set Expiration time to the link?

Here is my code:


public partial class ForgotPassword : System.Web.UI.Page
{
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["conn"].ToString());
SqlDataAdapter da,da1;
SqlCommand cmd;
DataSet ds,ds1;
Guid g = Guid.NewGuid();
protected void Page_Load(object sender, EventArgs e)
{

}
protected void btnSubmit_Click(object sender, EventArgs e)
{
string guid = g.ToString();
DateTime d = DateTime.Now;
//Response.Write(d.ToString());

string s = "insert into tblForgotPassword(Email, ActivationCode, TimeDuration,Status) values('"+txtemail.Text+"','"+guid+"','"+d.ToString()+"','Pending')";
da1 = new SqlDataAdapter(s,con);
ds1 = new DataSet();
da1.Fill(ds1);

try
{
DataSet ds = new DataSet();

{
con.Open();
SqlCommand cmd = new SqlCommand("SELECT RegId,UserName,Password FROM tblRegistration Where Email= '" + txtemail.Text.Trim() + "'", con);
SqlDataAdapter da = new SqlDataAdapter(cmd);
da.Fill(ds);
con.Close();
Session["email"] = txtemail.Text;
}



if (ds.Tables[0].Rows.Count > 0)
{
Session["username"] = ds.Tables[0].Rows[0]["UserName"];
Session["RegId"] = ds.Tables[0].Rows[0]["RegId"];
MailMessage Msg = new MailMessage();
// Sender e-mail address.
Msg.From = new MailAddress(txtemail.Text);
// Recipient e-mail address.
Msg.To.Add(txtemail.Text);
Msg.Subject = "Your Password Details";
//Msg.Body = "Hi,
Please check your Login Detailss

Your Username: " + ds.Tables[0].Rows[0]["UserName"] + "

Your Password: " + ds.Tables[0].Rows[0]["Password"] + "

";

string Body = "Hi, this mail is to change your password" +
"To Change your Password Please click below
";
Body += "http://localhost:1354/socialnetworksite/ResetPassword.aspx?uid=" + g.ToString();

Msg.Body = Body;

Msg.IsBodyHtml = true;
// your remote SMTP server IP.
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.Credentials = new System.Net.NetworkCredential("abc@gmail.com", "abc");
smtp.EnableSsl = true;
smtp.Send(Msg);
//Msg = null;
lbltxt.Text = "Your Password Details Sent to your mail";
// Clear the textbox valuess
txtemail.Text = "";
}
else
{
lbltxt.Text = "The Email you entered not exists.";
}
}
catch (Exception ex)
{
Console.WriteLine("{0} Exception caught.", ex);
}
}
public string Encryptdata(string password)
{
string strmsg = string.Empty;
byte[] encode = new byte[password.Length];
encode = Encoding.UTF8.GetBytes(password);
strmsg = Convert.ToBase64String(encode);
return strmsg;
}
}