Unfortunately almost never, otherwise I’d have more of them by now. 
And every time they do pop up, it’s a different version.
I tend to save all pictures of giant Pikachu cards I come across. You can find all of those in my Pikachu WTB thread (they’re at the very bottom, browser Ctrl+F for ‘giant’).
But most of them are store/event displays, which rarely pop up for sale. Only the Shadowless Base Pikachu (second picture) and the Pikachu VMAX inside the cardboard I’ve seen available for sale before - and the seller of the first one only wanted to sell 10+ giant cards as set without splitting up, and the other was already sold when I first saw it.
I’m glad I came across the Italian one I have. Also got a giant Italian Lightning Energy from the same seller. 
Greetz,
Quuador