Buying grocery's how long do they leave food on the shelf before they restock?

Tweet

I cant tell you how many times I ended up returning food to the store because it had spoiled and was out dated, the things is some stores put new dates over the old ones, and I dont get why some stores do that, I mean dont they understand that if someone eats something rancid or spoiled they can get sick, I think that the store should have stiffer rules when it comes to restocking. So anyway I'm wondering how long do the grocery stores keep food items on the shelf before they change them and restock, and have you ever accidentally bought something spoiled and had to throw it away or return it?