Duration is a single word to connote length of time.
Whereas, the question "What is the delivery time?" is ambiguous as to whether the question is
- When will it arrive?
- How long does it take to arrive?
Using the word shipment is more appropriate as it connotes the start of delivery to the delivery at door step.
What is the shipment duration?
How long would it take to ship the item?
There is a difference between item, shipment and product
When you ask "how long, or duration, of shipment of item", you are asking about one particular item.
When you ask "duration of shipment of product", you are asking about the general time that it takes to ship any item of that product, presuming you are interested in having an item of a product shipped to you more than once.
You could also ask for "duration of a shipment of product with a quantity of 5000". which would be asking for a particular shipment of 5000 in quantity.
However, if you are purchasing a large quantity, like say 20k items, it might take them two weeks to manufacture and prepare the product, as they may not have on hand that quantity. In which case it would be delivery duration, of an order. Using the word order when asking for duration, clearly distinguishes itself as the time between placing an order until time of arrival at doorstep.
- What is the duration to deliver an order of 20 thousand units?
- What is the duration to deliver an order of a single unit?
In summary, the keywords you need to consider and differentiate are