I think a lot of it depends on when/where you are flying.

I've had one diversion in ~100 trips--there was a typhoon near our destination. It was far enough inland that it didn't get flooded or anything but there must have been too much wind or something. We weren't told why beyond simply the typhoon. I would say it was roughly 6 hours later when we did get there. (Our original flight simply skipped the city and went on to it's next scheduled location and they then put us on the next flight going the other way which was held on the ground for a while before they let us fly.)