I drink a pint of beer and then water only if I'm getting paid or hoping for bookings. If I'm playing for fun I do about a pint of best per two hours. When I start to lose concentration I stop and grab some tea or water and lay off the beer for a while. This way I can drink all day and still sing and play after an all-day session.
I hate pretentious pratts that get stewed and become foul mouthed and abusive at the end of an evening. Trouble is, there's a few of them out there and they never mature with age.