Under the OrderTypes when setting your Wait Time there is an additional setting called Large Order Wait Time.

The only time that a Large Order Wait Time will be used is if there is a Large Order Threshold $$ Amount set up. This tells OrderCloud if an order reaches the subtotal of what is set in the Large Order Threshold Amount it will use the Wait Time set in the Large Order Wait Time in place of the Standard Wait Time

In the Example below the Standard Wait Time is set to 15 minutes. The Large Order Threshold $$ Amount is set to $150 and the Large Order Wait Time is set to 30 minutes. If my customer places an order with a subtotal of $150 or more then OrderCloud will use the 30 minute wait time for the order.