It's fairly quick to get a MO replaced, I'd say two weeks.
I had a curious situation recently where I mailed a bank a WU MO for $995.07 and they posted it for $995.01. They said that as they read it, it was $995.01. I found the receipt and it's clearly printed $995.07. I told them that I really didn't want to fuss over 6 cents, but that it was definitely $995.07 and if they ended up with 6 cents extra and needed to reconcile their books they could give me the 6 cents.