This depends on your ecommerce store.

Shopify and BicCommerce inventory updates are in 'real time' - if you change the inventory in Shopify/BigCommerce, CatchFeeder receives notification of the change and then updates Catch.

Magento inventory is updated every 20 minutes.

WooCommerce inventory is updated every 24 hours (overnight with the full product sync).

More details are available in the Syncing FAQs