We had beer taps at my previous job, and nobody drank during business hours even though they could have. Any work past 5 or so might have been under the influence from time to time, but we really only drank in any quantity after quittin' time.
Pretty much the same at my current place, most people won't use the kegs until its an appropriate time. I've had a beer at lunch or late afternoon a few times though and its never made me feel less productive. Sipping on a beer while coding is a nice experience imo.