I am wondering how often the Trello webcal page is updated. I would like to have my Trello due dates appear in my Google Calendar, but I need to ensure that the dates are being propagated to Google Calendar.
Google Calendar updates "every few hours." Unfortunately, it's not a configurable setting. See How often does Google Calendar update its "Other Calendar" feeds?
The Trello iCalendar feed requests that clients refresh the feed hourly, but they have no obligation to do so (and, indeed, Google Calendar ignores that request). You can see the last time the feed was updated in each event's description under "Feed Last Updated."(Source: I'm the Trello dev who wrote the iCalendar feed)