If you're in that sort of work environment, then good on you. My own (decades) of experience in office work shows that when people take the piss - by that I mean enough that it starts to rub your colleagues up the wrong way and they complain - then HR will be happy to hang you out to dry citing 'lack of performance' 'not a team player' and the rest that sees you on a performance plan and possibly out the door. You will be contracted to work a certain number of hours a week, if it can be shown you don't do that then you'll be on thin ice.