It depends on how far behind the orders are. You can click on a specific item's name from the buy/sell page, and see more detailed market information about it. You can further look at the charts to check the consistency of the numbers you are seeing. What you are looking for is the demand over supply, which gives you the number of game months the current order backlog stretches into.
When I did the fortifications in my slave, it took thirteen real life days for the order to fill.