G. Mark’s Answer
You first need to understand what psychology is. It's the science of how people think, respond to and understand their environment and how they interact with each other. Considering that humans are a social species, most of what they do is connected to other humans. So the obvious answer is that everyone uses psychology all the time.
Now, let's assume that you are referring to an active, willful use of psychology for a particular person, like myself. In my personal case, I'd have to say "all the time". I need to understand what people do, what they want to achieve, and what they want from me. It's basically the "lube" that allows people to work together better. So knowing more about why people do what they do and what they want and what will make them happier is pretty much the most important thing you can do.
So, that being said, I'd have to simply answer, "All the time."