flight endurance

I have an Elev-8 (V2) quad that I had modified - replaced the stock motors with Tarot 4006 620 KV motors, driving carbon 1355 props. I'm using an APM 2.6 flight controller with external GPS, telemetry, Tarot 2D gimbal and a TS832 600 mW video transmitter. The full up weight is 3.0 lbs: I...