Googles "last updated" number of hours ago snippet

[full house]

is there a fix time?I don't think so... google has flexible algorithm that hard to figure out and I think won't stick to a fix one.

[deepsand]

Could that also be the latest time Google fetch the page from sitemap?

How or when a resource was discovered is neither relevant nor necessarily limited to a single source/method.

The only thing that is material and relevant is when the data were acquired, which would be the cache date/time stamp.