How often does iCloud sync bookmarks, between iPhone 4 and iPad 2
It seems that it may be tied to the Fetch New Data setting in Mail, Contacts, Calendars.

When I change that setting to Push, the bookmarks are sent out straight away.

What I don't understand is why the Photo Stream pushes out changes immediately, even if 'Fetch New Data' is set to 'Every 15 Minutes', but the Bookmarks (which appear to have nothing to do with 'Mail, Contacts, Calendars', are not.

Later:

I've just seen the "Advanced" setting in "Fetch New Data". This seems to have a manual override 

With Fetch New Data set to "15 min", (and Push is Off), and iCloud set to "Push", I expected to have the bookmark pushed out straight away.

But this doesn't happen: I also have to turn the main "Fetch New Data" setting to Push, in order to have the bookmarks updated immediately.