If there was a link between the Project and its Template, you could go retrieve the correct Billing Schedule from there during your UE, but I'm not seeing such a link. Given that, I think the best architecture is going to be either a scheduled or map/reduce script that simply scans for SO line items that are associated to Projects but don't yet have a Billing Schedule. Even if

setTimeout

worked server-side, I would not advocate for that approach as it would add that length of time to every single Project save. I think a script running regularly in the background to make sure Projects and their SOs stay in sync will be cleaner, more performant, and less intrusive. You can tweak the timing of how often that script runs based on the needs for the data to be updated and how many queues you have available