Frequency varies. My first couple kegs were polished off in less than a week each, but since I started brewing weekly I have way more beer than I can go through...it's gotten to the point where I don't have enough fermentation vessels to keep up, much less cornies. Been sans a brew day for two weeks because of it, I'm lonesome when I'm not brewing! I do three week primaries and a week force carbing in the keg so my grain to glass is about a month, but even with two taps on my kegerator I feel like two brews a month is pushing it these days.
Then again, there's all that mead and skeeter pee to keep people occupied...