I am a male 65 years old. A type 2 diabetic for last 7years. Take metformin 500mgs twice a day after breakfast and dinner. Hba1c is 6. When to insulin?

No need for Insulin. With a good hba1c of 6%, you are fine. Just stay on the metformin. Most diabetics may need more agents and Insulin with time but there is no formular to predict when you will progress further. Keep to a healthy lifestyle - exercise, eat well and keep your weight down. That may help to delay progression (no guarantees though).
Not now. You certainly don't need it now, but if you're taking metformin, the most appropriate dose (if your kidney function is normal and you can tolerate it intestinally) is 1000 mg twice daily. Ask you doctor. There are beneficial effects of metformin other than glucose lowering. If you increase the dose, do it in weekly intervals.