So to promote this from the commends - as a linux user I'd set a line in the cron table that increments a counter every minute and then when you got up to your particular limit to ring an alarm.
Something I've done occasionally is simply not taken my laptop charger with me - built in limit on your work. (A handy bonus thing here is if one's other half objects to bringing a laptop on a holiday - not taking the power supply is a wonderful compromise...)
Oh - the other thing... So address the problem at a differently level - when I know I have lots of "other" stuff to do I convert to a kind of standing-desk arrangement - that keeps me pottering around the house on a regular basis and it great for avoiding the 'internet stupor' I fall into when I know I have to get up and do stuff around the house.