I'm pretty fastidious about keeping the moving parts clean. Fork stanchions get a wipedown with a shop rag after every ride, drivetrain gets cleaned and lubed after every wet ride, or maybe every 2-3 rides if it's been dry. Other parts (brakes, derailleurs, etc...) are inspected after every ride and cleaned as needed.
Other than that, everything else gets cleaned when I get tired of sweeping up the trail of dust the bike leaves all over the garage. Even then I'll usually just let the dirt dry and knock it off with a brush. Sometimes I'll rinse it with the garden hose. It never gets scrubbed with soap and water unless I'm planning some maintenance, then I'll clean up the part I'll be working on.