Thanks for asking about this. Stock history was recently updated. I’m waiting to hear back from the engineer who made the update about how it now works.
It is an undocumented end point, this means that the way it works may not be entirely as expected!
The stock_history end point is just a feed of the latest stock changes and the entities that caused those changes.
These changes will be fairly instant, however could be up to a couple minutes delay at times depending on the back-end worker that is processing the changes.
The reason why we haven’t formally exposed this endpoint is that its quite advanced. Before we expose it fully (and provide better support and documentation for it) we need to check all the code and make sure that it runs consistently, predictably and reliably.