there are 7 people in my office and i'm the only hardcore, everyday, 12 month bike commuter. another guy rides in about 1 or 2 days/week on average april-october. a couple others are even less frequent summer-only riders.
so this is a hard percentage to calculate because we first need to agree on what the minimum threshold of a "bike commuter" is.
my office is in downtown Evanston (Chicagoland).