Old thread, but a subject near & dear (so to speak). Personally, my rule is no more than 2 pints while the gig is on - to wet the whistle between songs.
I used to work with a drummer who was very impressive for the first half of every gig, then gradually became less so as the night wore on. The fact that he doubled on vocals didn't help much either. He serendipitously ended up removing himself from the picture before it became too big a problem, but I could see it comin' eventually.
Blues guys, smilin' at me / Nothin' but blues guys do I see..