It depends...
When I'm slowing down and in other situations with zero throttle = no lugging or other issues even at the lowest revs. If I open it up fully, it needs to be north of 4,000/4,500. Soft gradual roll on is fine from 2,000
These bikes work best at +/- 6,000. Best balance, least vibration, best response, best mileage
Maybe cruising with a steady 20-30% throttle at 4,500 is also ok...