In Flow designer, navigate to the catalog item's flow and select the 3 dots in the top right hand corner and go to 'Flow Stages'.
Add a new stage and name it appropriately (eg 'Fulfillment'), setting the Duration to whatever you want your delivery time to be.
This value will now display in the catalog item form.
How to set catalog item Deliver time using Flow Designer for Service Catalog
Working Code Asked question August 19, 2024