It also depends on how close you intend to get to the cylinder. For pipes I've gotten away with 3 sided cylinders and some creative bump/displace/normal mapping.

For sturdier modeling I ussualy go with 9, 18 or 36 sides so I have a nice clean 40, 20, or 10 degrees of arc per polygon.

mo·nop·o·ly  [muh-nop-uh-lee]
noun, plural mo·nop·o·lies.
1. exclusive control of a commodity or service in a particular market,
or a control that makes possible the manipulation of prices