Target shooters in my neck of the woods buy by the (250 count) flat, usually when Winchester or Remington are offering rebates, Walmart Federal bargain packs, or other brands like Estates and Rios when they go on sale. Hunters usually keep less ammo for game like Turkey and Goose, although dove hunters buy flats of whatever is cheap and goes bang (dove hunters shoot a lot of shells!) With expensive steel shot mandated for waterfowl and smaller takes permitted, duck loads aren't the biggest sellers (one season---one box of shells seems to be the mantra)
I've currently got two flats of Remington field loads, a flat of Winchester AAs and half a flat of old Federal Leagues for clays, two more flats of lead hunting loads(Federal and Winchester Super X) and a box of Estate 4 Buck, mostly #8 for targets and #6 for hunting---all bought on sale except for the Estate 4 Buck. The target loads will all be spent by this autumn while the flats of hunting loads might take a while to shoot up (but not too long awhile, as lead upland game loads will be banned for hunting in California by 2017 as I understand it )
What sort of supply of shotgun cartridges do you feel comfortable keeping for your shooting sports? How long does it usually take for you to "turn them over?"