24x7
I expect to be available out of hours since we're a very small department supporting a group with companies in Australia and New Zealand amongst others. However I would only respond to emergencies, anything routine waits until my normal work time (which is longer than office hours but that's my choice).
I also will pick up things when on leave if I have to, it reduces the stress when I return (having once had an employer cause me a week's work rebuilding a Unix box after they let an idiot of a service engineer at it while I was on holiday I'd rather know and react to major issues).