As a freelancer/contractor/consultant I could move as often as I liked without getting a CV stain, but I have tended to spend multiple years (even decades) on a

As a freelancer/contractor/consultant I could move as often as I liked without getting a CV stain, but I have tended to spend multiple years (even decades) on any one task (often overlapping), whether a client or my own start-ups. So, I don't think there is a 'right' amount of time.