One hour and a usb live stick should be all that is required to let Grandma try out KDE, Gnome, XFCE, etc.
Let her pick, she is the one that has to actually use it after all.
Lol - tried that one with my mother (age 78). After about two hours of my showing here the different desktops, she finally flat out told me to stop wasting her time, and install something with the 6 applications that she gave a damn about.
So now she has a Debian Wheezy system with LXDE (it's an older machine), and 6 application launch buttons. She has no idea whatsoever how to change *anything* on that machine, and couldn't navigate her way to any of the applications via the normal menu.
And she loves it.
The moral - don't try and turn Gradma into a nerd. You probably won't succeed, and she won't be very happy with you for trying. Instead, aim for something like a "walled garden", with you being the gardener...