If you choose the download only option, that's exactly what it will do - just download it - it won't actually update it.
If you want the OS to be updated as well, you need to choose the download and update option.
It will take as long as it takes. There are no shortcuts to a faster update.
There is no answer to how long it takes. The speed of your internet connection is a major component of the process, which we cannot know. As Julian says, it will take as long as it takes, no matter how long you spend trying to guess.