Time until creation of the next item in the source

Ladies and Gentlemen,
I am building a model that uses a number of sources, some with a set interarrival times and some - schedule driven.
In order to schedule the materials handling operations, when the item is released by one of the sources into the queue, I need to know how long until the next one is created. This will allow choosing between waiting to fill the transporter to a capacity or dispatching a partial transport.

What in your opinion is the least hacky way of doing so?
Thanks!
Vadim

If you're not stopping the sources these examples should work - where the next arrival is put into a label on the source.