Besides getting bad flat spots, I'll change em when they start feeling all laggy and sluggish, which is usually when they've already shrunk down in size quite a bit already. When it takes you ten pushes to get down the sidewalk when it only used to take you five, that's a good sign that it's time. Sometimes its only after a few months, other times its half a year or more, it depends on the wheels. I don't have a car and use my board to skate everywhere I need to go though, and I'm constantly out and about, so I tend to go through wheels faster than most people.