From my personal observation and from the occasional SE dev statement, 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, 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?