From my personal observation and from [the occasional SE dev statement][1], many items update every 60 seconds (cached at server).

Also the page AJAX updates every 60 seconds.  
(using JS like: `setInterval(updateRelativeDates, 60000);`)

So, per [sampling theory][2], sampling more than once every 30 seconds gives diminishing returns.

Personally, I would just reload every 59 to 61 seconds.  Since that's the same interval that the page uses, you can hardly get in trouble, right?

  [1]: http://stackapps.com/a/3544/7653
  [2]: http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem