It depends on how often you ride, if you ride in the rain, etc. Your chain needs lube when the sound if it going across your chainring and cog gets louder. Your chain needs cleaning if it looks really dirty, or if you can hear the grit in it.

Either get a chain cleaner like this one or take it off and shake it up in some simple green, kerosene, spirits or whatever else you use. Add your lube of choice conservatively -- I put 2 drops of oil on every roller, let dry, wipe off excess.

Your chain needs replacing when 12 links measure more than 12 1/8".