I work as a lawyer for an international firm (working in England). I have always maintained that my work mobile will be on during working hours if I am out and about, but otherwise will be turned off. I do not read or answer emails on holiday. My weekends are time for me and my family.
These points have always been made clear to my immediate superior, and as I am reasonably good at my job it has never been an issue. However I see with colleagues that if you open the door even a slight crack on any of these points then it becomes immediately expected that it is the norm and you are expected to do it at any point in the future.
You have more power than you think to define your own boundaries – unless you truly love your job you work to live, you don't live to work. Nobody has the deathbed regret that they wish they had spent more time at work rather than with their family.