in your NS file:

  	$ns at 2000.0 "$ns halt" (or terminate)
  or
  	$ns at 2000.0 "$ns swapout"

  The first causes the experiment to terminate, the later causes it to
  swap out. The units are seconds, as are all "at" statements.

  NOTE: You can use halt (terminate) to cancel a batch experiment, but you
	cannot schedule a swapout.