It seems to average 24 hours, though it can go longer than that. I've even see it happen all but instantly. In one very small group of products I'd uploaded covers for on the same day, two gained their covers within 24 hours and one lagged behind by a few days. It certainly must be an indexer doing the work, so all I can figure is that it has something to do with when it's passing by and then when it comes around again.
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