I think it depends on how far you are into the game. Like when you first get to the wild area, everything is weak, with only 1 -2 stars. But later it becomes 3-4 stars. And I think once you beat all the gym leaders, it becomes 4-5 stars (sometimes 3). I usually only find 5 star raids, and some with 4 stars. Take this info with a grain of salt though.