Handling Time (also known as Fulfillment Latency) is the number of business days between when you receive an order on Amazon and when you ship it. The default value on Amazon is one to two business days. More about this feature can be found here: http://wiki.sellercloud.com/channel-management/amazon/amazonproperties/fulfillmentlatency
You can now set the handling time for your products on a Vendor level when Spark Shipping runs inventory updates:
Go to Vendor -> Update Vendor Settings -> Advanced Settings -> Handling Time. Spark Shipping sets the time to 2 days by default, but you are free to update it to the vendor's Fulfillment Latency as you see fit.
*Please note that changing the Handling Time will take effect in Amazon when the next inventory update runs. If a product's quantity does not change, handling time will not change either. It will only update when there is an update to be made for a product.