This is the maintenance schedule I try to stick to for my commuter bike:
Once a week pump up the tires
Once a month clean and lube chain
Every 2-3 months clean the entire bike, including wiping the crud off the rims (bike has rims brakes), and cleaning the entire drivetrain
Once a year replace the chain and brake pads
Every 1 1/2 to 2 years clean and repack wheel hubs and bearings in clipless pedals, replace bar tape
When I think about it, usually once every 3 months, hold the front wheel still and check that the stem doesn't slip on the steerer tube when I turn the handlebars