The one time we won - from oceanview to balcony, we were notified about a month out on a medium bid and it was a sold out sailing. The next two cruises we went on (both oceanview to balcony bids, both max), we were notified that we didn't win the day before sailing, also both sold out sailings.
We'll see how successful the one for our cruise in two months is, right now there's lots of rooms available to upgrade into. I'm not terribly hopeful though, this time we're trying to go from JS to either GS or OS and I don't feel like paying anywhere near the maximum bid because I'm more than okay with the rooms we've got.