In scientific research CoQ10 is typically used in doses of 150 to 200 mg. It’s safe to take far more than this, but it’s not clear whether there would be any additional benefit to take greater doses.
When dosing an supplement like CoQ10, you're bound to get great outcomes when following clinical research conventions closely, so the best evidence recommends that 150 to 200 mg or so is a decent portion to begin with.