You should restock your inventory when you are low on that specific product if it makes sense to. For instance, the listing may change from the time you started selling to the time you are almost sold out of that item. Also, competition may have increased, the price may have dropped, or you supplier could be out of stock of that item. If the conditions are right and you have access to the product then restocking it can help maintain a steady stream of revenue and increase your overall IPI (Inventory Performance Index).
If you are in seller central and go to you Manage Inventory > Inventory Dashboard >Restock Inventory, then Amazon will show you all the products that need to be restocked and how much you should buy. If you are unsure of the amount to purchase, then this is a good guide to follow.
In addition, Amazon gives you the recommended date of when to actually send another shipment of the product in.
When should I restock?
Is there a rule for replenishing inventory?