Inventory is updated between 20 minutes and up to 2 or 3 hours between Magento, Omnivore and Marketplaces such as eBay, Amazon etc. The frequency of inventory updates depends on the number of products. For example, if you have 50 products, the inventory will update more frequently - around every 20 minutes throughout the day. If you have over 5000 products it will be every 2-3 hours.