Lance: You control those things via the Aweber interface.
Start by logging in. Click the web forms tab. Click the name of your lightbox form to edit it. Now, in the editor view, look for a blue tab on the right side (above the WYSIWYG view of your form) called "Advanced." Click it and it will expand down. It should give you fields for "Display Options," "Delay," and "Recurrence."
Set the delay to whatever you want (20 seconds) and set the recurrence to "Display Once." Now you just have to save it.
Protip: If you want to test it out, the Chrome incognito mode it useful (doesn't save cookies so you can close/re-open a new one for repeated testing).
Hope this helps.