Depends on the kind of format you do. You can do a "quick format" which simply changes the image's table of contents to read that the whole drive is empty, but leaves all the data essentially in the same place it was until it writes over it. That type of format literally takes a few seconds, then done! Or you can do a complete format, in which case the entire contents of the disk will be reformated to be 0's, and that can take a hell of a long time depending on the size and speed of the drive. But what you describe is a bit weird, you get to 99%, and then stall there for a long time. Sounds like something might be wrong, but I'd give it some time to recover. Can you hear the drive working or is it silent?