Set the timer interval on 5 minutes
<asp:timer id="Timer1" ontick="Timer1_Tick" runat="server" interval="300000" xmlns:asp="#unknown" />
Then make the last update like
protected void Timer2_Tick(object sender, EventArgs e)
Label1.Text = "Last Updated at : " + DateTime.Now.AddMinutes(-5).ToLongTimeString();
EDIT:
To stop the timer:
protected void Timer2_Tick(object sender, EventArgs e)
Label1.Text = "Last Updated at : " + DateTime.Now.AddMinutes(-5).ToLongTimeString();
timer1.Enabled = false;
Cheers