Looking through my past successful/unsuccessful JAL upgrade bids, it looks like they cleared (or failed) pretty consistently about 24 hrs before departure.

I always bid about 5-10% higher than the minimum and get the upgrade about 50% of the time. If I *really* want an upgrade, I'll do it through mileage—confirming before booking—a seat I like. I usually only bid if I have a PE seat I don't like or for one of my kids.