I've had a green one with blue liquid and a black base since I was about 13.

I leave it on for days at a time and it's fine. Only problems I had was the "lava" stopped swirling around and settled into a blob at the bottom after about the second day straight.

Bit off subject but I always have these crazy dreams when I sleep with it on. Know it sounds like bullshit but it's true and the dreams never fail to come about when I have it on...