You can set a timeout to wait a few seconds before firing an event. It can be useful to track engaged users for instance!
In this example, you might want to track a user who has been here for at least 5 seconds:
<scripttype="text/javascript">
var timer = 5;
setTimeout(function() {
fbq('track', 'Lead');
}, timer * 1000);
</script>
Here, timer is a variable used to define the number of seconds the event will wait until getting fired (timer * 1000 at the end is to convert this value to seconds).