Use it whenever you use that lens. The hood does two major things for you. The first is that it protects your lens in case you bump it up against something or drop it. The second is that it helps block stray light from hitting your lens. If you have a bright light (like the sun) that is not in your picture but is shining on your lens, it can wash our the picture (make all of your dark areas much lighter than they should be). The lens hood helps protect against that.
I use my hoods almost every time I use my camera. When they aren't in use, I flip them backwards and screw them onto the lens that way. It's an easy way to keep track of the hood and not use any more storage space than necessary.
So when don't I use a hood? Sometimes when I'm pressing the lens directly against a window, I take off the hood. I sometimes remove it when I want to get really, really close to something. A good example is when I'm shooting through a screen. I'll put the camera as close to the screen as possible so that it will be so completely out of focus that it isn't visible at all. I've also taken off my lens hood when I wanted to be very inconspicuous because lens hoods tend to draw attention.