It is not posted and nowhere to be found - and I too am keenly interested in the answer.

The best answers I have found have been in the vikings wikis. The principles explained there appear to be the same, albeit the actual values aren't.

One of those principles, aside from as you noted the higher the level the faster the yielding, is that yielding from fastest to slowest is: food, stone and lumber, iron, and silver. [Even though iron and silver camps are 2/3 the size of the other camps.] I think I've seen it said silver is twice as slow as stone.

The rate is further convoluted with whether the hero and inquisitor is or is not present on the march, and the equipment type. [High Resource Yield equipment seems to have the greatest impact. Sadly, only if it is being worn on that march.]

Given so many variables, such as jewels, this is a very hard number to pin down. [And if we could, then we could know how many troops to send to have them return within a particular time period. Such as 1h.]

This is all why I have made feature requests in the Technical Support forum to have duration shown beside capacity.

[Even two consecutive and consistent marches can have different durations. A boost could expire, or any number of other factors. Let alone expire -during- the march.]