I wipe down my playfields with a clean cloth weekly but every six months I'll take the playfields apart, clean all the corners and plastic parts, and apply wax to the playfield. I lay down three coats of carnuba wax before re-assembling and then let it rest for a day (which may not be necessary). This is also a good time to change any bulbs or rings and replace the batteries and balls.
It takes about 6 to 8 hours per machine so I pick a long weekend - Thanksgiving and Memorial Day work well. (That's November and May for our friends outside the US )