We ran a test of info-gathering popups on one of my sites a little while back. We split tested using various triggers.

popup after 5 seconds:

- 73.5% left the page without closing the popup.

popup after 10 seconds:

- 75.8% left the page without closing the popup.

popup triggered when the user scrolled near the bottom of the page:

- 21.5% left the page without closing the popup.

The scroll trigger was pretty useless though because only 2.6% of users scrolled low enough to trigger it.