You will never see 20 mins with that setup, you will need to increase your cell to 4, drop the KV of the motors and increase your props more. Then you may see them times with the options added to it... "MAY".
I run a disco pro with 900KV, 4s 4500mAh, 500mw 2.4Ghz VTX, ImmersionRC EzUHF Diplex 8 channel RX, Gimbal obviously with GoPro 3, ImmersionRC EzOSD, Flytrex all on 10" Graupners. I get around 12 minutes or so, I could probably push it a little more but I do not want to cause early failure of my batteries.
I had a F550, I thought it would lift anything!, how wrong was I, the motors are not up to the work, the frame is heavy and mine struggled to get off the ground with a GoPro 2, landing skids and dual 2500mAh 3s batteries.