Why does it take so long to develop interfaces but actual time work is so much shorter?

That is a really great question.  If you have never developed an interface, it can be frustrating when it takes so long.  Most of the time, it is "Hurry Up and Wait!!"  Usually, there are a number of people who are responsible for a portion of the interface setup.  If it is a brand new vendor interface, most likely, the VPN needs to be set up.  This takes both an internal network engineer as well as the vendor's network engineer.  There is the development of the interface on the sending system, the interface engine, and the receiving system.  Quite often, it is waiting for the next resource to free up.  Once the interface is developed, there is the testing and waiting for resources to test the sending system and the receiving system.  There can be times that the development can be more involved based upon what is being sent and what is expected to be received.  Based upon my experience, that is only about 1/3 of the interfaces I have developed.