What is the maximum allowed expiration time for a Google notification channel?

Question

I can't find any documentation that answers this question. Specifically, I'm using the calendar API: https://developers.google.com/google-apps/calendar/v3/push

I know that the expiration time is supposed to be the number of elapsed milliseconds since EPOCH. Currently, I have it expire in a week, but I want it to be longer - as long as possible.

$chan = new Google_Service_Calendar_Channel();
$chan->setId($channel_id);
$chan->setToken("blahblahblah");
$chan->setType("web_hook");
$chan->setAddress("https://www.mydomain.com/catchPush/");
$chan->setExpiration(strtotime("+1 week")*1000);
$watch = $cal_client->events->watch($calendar_id, $chan);

Answer 1

The maximum TTL appears to be one month.

Try setting the expiration to one year, and monitor the expiration Google returns in the result.