Depending on the speed of your internet connection, getting Windows 8.1 may take a little while as it is several GBytes in size - not too bad on a fibre connection but could be a couple of hours on a slow broadband connection. The actual installation process itself depends upon the speed of the computer and whether you are doing an upgrade or a clean install. One thing to watch out with the latter is that the installation goes out of its way to give the impression that you need a Microsoft account or, failing that, an email address to sign-in with. In fact you don't, and by being persistent you can get past it. If you are in a business or school environment you may not want to be messing with Microsoft accounts and such.
Given that the version number has only gone from 8.0 to 8.1 there aren't, as you might expect, many differences. At one stage Microsoft were touting the return of the Start Button. Well, it is back, but all it does is take you to the Start Screen and there is still no Start Menu (you still need something like Classic Shell for this):
Other than that, there's not much. The full-screen Apps have now been tweaked so you can run two side-by-side. Some of them are quite pretty, but the whole concept is still daft to most people. There are additional personalisation options for the Start Screen and lock screens. Er, and that's about it.
Microsoft have used this opportunity to tweak a few things here and there. There's certainly nothing wrong with Windows 8.1, but nor is there anything to particularly get excited about either. If you like the Windows 8 approach, this is more of the same. And if you don't, this won't convince you!