When I was a child, it was customary to wait until Shavuot, which is six weeks after the conclusion of Pesach. This date was a “guesstimate” of how long it would take for the stock of chametz that had been in the store during Pesach to be depleted. Today, it is generally assumed that the inventory in a major supermarket is sold much more quickly; shelves are generally restocked on a daily basis. Still, supermarket chains maintain large inventories of products in warehouses for distribution in individual stores, and it is necessary to calculate the turn-around time from the warehouse delivery until the purchase by the customer in the actual store.
Rav Moshe writes (Iggerot Moshe, OC 4:96) that it is permissible to purchase chametz from a supermarket at the point in time when there is a 50 percent possibility that the supermarket purchased the chametz after Pesach. Since chametz sheavar alav haPesach is a rabbinic (and not a Biblical) injunction, one can rely on a principle known as “safek derabbanan lekula” (one can be lenient when it is uncertain if a rabbinic restriction applies), and therefore shop freely in the store.
The question is, When can one legitimately say there is a 50 percent chance that the chametz on the supermarket shelf was purchased by the store after the conclusion of Pesach? How long does it take to establish a reasonable doubt? It is difficult to give a precise cut-off date. Communal rabbis generally tell their congregants when they feel comfortable purchasing chametz, and my impression is that Lag B’Omer (which is twenty-five days after the conclusion of Pesach) is a safe time.
