For me it just depends on the piece really. I have a sterling silver bangle that seems to need polishing all the time, but I have a cheaper sterling silver necklace that hardly ever needs polishing.
pieces i wear frequently do not need polishing because i wear them. those i don't wear often are in a jewelry box that is lined with some fabric that prevents tarnish and/or are also in plastic bags that helps to keep the air out. seriously, pieces warn with frequency should not need polishing.
I'm with MZ above, stuff I am not wearing, is now stored in zip loc bags and the items I do wear, don't seem to be tarnishing.
My DD has a few pieces that she does not wear regularly and being 18 - they wont be polished unless dear mom offers! So... I do... because I'm not a fan of the 'dirty' look - love it shiny! I'd say maybe every 3 months or so??? (but I'm pretty forgetful so that could be every 6 months... not really firm on the time frame!)
I have a few rhodium plated pieces too. They are very easy since they don't need special care. I do polish my pieces when they start to look dull and I have just started to seal some of my silver pieces in plastic with little anti tarnish strips. I would love to have anti tarnish fabric in my jewelry box though. Great idea.