As long as you want it to last for!
2ml = 500mg per week. I have been on that much for 12 weeks and have just upped it to 750mg (3ml) a week for the next 2 weeks purely to get rid of the stuff before I move.
It's entirly up to you however a good cycle of 8 - 10 weeks is adequate as it will still remain active in your body for 10 - 14 days after you finish. Therefore 10 weeks on = 12 weeks.
Are you splitting your doses to a 1ml jab twice a week?