The answer to this question is less simple than gasoline or oil. Some sources will tell you that you should change it out every seventy-five thousand to hundred-thousand miles while some simply say that it should be changed every two years. The reality is, there's no real definitive answer and even the car manufacturers vary in their opinions.
However, fear not because no source is necessarily completely incorrect (or completely correct for that matter). The best advice that could possibly be given would be to meet somewhere in the middle. Either every two years or every seventy-five thousand miles (whichever comes first!), you should change your power steering fluid out.
Changing out your power steering fluid is incredibly important because of how the power steering pumps function. Power steering pumps are hydraulic pumps which put out upwards of sixteen-hundred pounds of pressure. Over years of use, the power steering fluid gets dirty along the way. Dirty fluids create vastly more friction, becoming abrasive. Dirty fluids can even go so far as to ruin the seals in the pump, or the seals on the rack, which can be incredibly expensive to replace.