How frequently do the search engines update? Do they all update at the same time? Are there any signs you can look for to let you know they're about to update?

I dont actually know this but I think that they are constantly trawling through pages. They do not have a set time say at 2am to start trawling. Maybe check on google to see how often the re-check pages

There's a sort of refresh time, I think... but they don't all do it at the same time.