how to use the timers?

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C#

i need to run a form for 5 hours from the first form load.
how can i check how long is it being started to run the form?
"time< 5" this condition is not acceptable,how can i change this in order to get the time value?
please help me.

private void mainpg_Load(object sender, EventArgs e)
{
DateTime time=timer1.Start();
if (time< 5)
{
//remain in the same form
main_test one = new main_test();
one.MdiParent = main.ActiveForm;
one.Show();
}
else if(time >= 5)
//exit from the application.
}