I reply to messages when I see them...whether that is after a second of it arriving to my phone or a few hours... doesn't make a difference who is texting me, whether it be family, friends or work, I'll reply when I see it.
As someone said above my phone is also always on silent with no vibrate...only time I take it off is if I am expecting an important phone call or I'm meeting up with mates and want to know if they're updating me of their whereabouts/change of plans etc.