Tooltips and lightboxes are on-page bits and bytes of contextual information that help the user understand what’s happening or what a term means now. The idea and expectation is that a short pause is needed to absorb what’s here so that what comes next will make more sense.
The three-and-thirty rule of thumb says that you should shoot for providing three seconds time for a user to absorbinformation.
A lightbox should take up about 30 seconds of your reader’s time. Keep in mind that people normally take a little more time to read a screen than a page, especially if they are learning something new.
(That was the first ten lines of Chaucer’s Canterbury Tales. Did you know that the world over, a “line” of poetry—that is, a few words and a pause—settles in at about three seconds?)*
*Stephen Pinker, The Blanks Slate: The Modern Denial of Human Nature
The droghte of Marche hath perced to the roote,
And bathed every veyne in swich licour,
Of which vertu engendred is the flour;
Whan Zephirus eek with his swete breeth
Inspired hath in every holt and heeth
The tendre croppes, and the yonge sonne
Hath in the Ram his halfe cours y-ronne,
And smale fowles maken melodye,
That slepen al the night with open ye,