To this end, I heartily recommend that any Gmail user in exile from the homeland enables the Sender Time Zone feature in Gmail Labs (ie. the thrilling antechamber where you can test-drive probably-great-maybe-rubbish beta propositions on your Gmail account). Not only does it show the local time an email was sent, it also gives the current time over there. Probably the most useful element is the little phone icon that appears next to a sender's name, which turns green during local 'business hours' (9am - 6pm) and red-for-danger outside of those. Whilst I'd argue they could afford to extend that to 9pm (which, strangely enough, this otherwise-useful Lifehacker article seems to believe they already have done), it's preposterously useful as an at-a-glance guide. Just make sure you've got your correct time zone set in your own Google account settings, obviously.
Sadly, the biggest issue still remains. Tell me - when IS the 'cut-off'?