Days! We've waited weeks before now. Our feed is uploaded daily. But the images can take ages to get refreshed. Even images flagged for low quality (for example because they weren't cut out on a white background) can take a long time to get refreshed after the issue has been addressed.
this is a rather old thread.
google may take a few weeks or several months or
more, to update changed images for a shopping-ad.
to help improve the refresh-rate, if possible, use a new file name, under
a new directory/folder, or add a random url-parameter, or all three; e.g.