How to use browser.wait() in zombie.js?
Zombie.js will by default wait untill all the scripts on your page have loaded and executed if they are waiting for document ready.
If I understand you correctly, your script will not execute til after 20 seconds of document ready. In that case Zombie has a function which will let you evaluate javascript in the context of the browser, so you can kick off your ajax code quicker if it is on a timer, and you do not want to wait for it.
Look at browser.evaluate(expr)
Another option would be to simply use a normal JavaScript timeout to wait 20 seconds, and then look at the DOM for the changes you are expecting.
setTimeout(function(){ browser.document.query("#interestingElement")}, 20*1000);