My experience is also about 72 hours, a little longer over weekends. I've never had flight miles post during a weekend, so if you stop the clock for the weekend 72 hours would be a good guess.

Last week I flew STL-DFW-SJD on Thursday, and the miles posted Monday morning. I flew back Wednesday and the miles have not yet posted, but I would be surprised if they don't post this Monday.

However, stickers withdrawn for upgrades are taken out very quickly, and that activity does post on weekends.