limited edition releases (i.e. the color stores, like rockocco and untamed..) are kept until they run out. occasionally, certain stores will get secondary shipments a few weeks after the initial run. items that don't sell are often RTV-ed (return to vendor) after a set amount of months, then MAC decides what to do with them. on average, i've noticed, the initial runs of collections usually last a week or two, with things starting to slowly sell out. sometimes, lower traffic volume counters will have things longer, and occasionally super popular items (i.e. the MSFs and pigments) will sell out in a couple days.
for example, i somehow managed to miss out on gold dusk pigment and the summerwear quad when they were released. i called about a billion MAC counters and freestanding stores, and couldn't get ahold of any. three weeks later i happened to walk into my nordstrom and they had gotten in a smaller second shipment, and i was able to get them. i was thrilled. what i can recommend is keep a list of MAC phone numbers in your area, and make friends with a MA or two. call in advance, if there's some doubt something will be in stock. usually, they're willing to hold an item for a day or so, as well.