You should get 6 min flights minimum, that is what I fly in idle up. A few things to check:
Is it cutting due to loss of voltage (bad batteries or charges)? Check the voltage before and after the flight. It should be over 11 volts both times.

Is something over heating?
Check your gear mesh like mentioned above with a piece of paper. It should go through the gears and get a "krinkle" but not tear.
See if your ESC, motor, or servos are hot. If you can keep your pinky on it for 10 seconds it is cool enough. Also check the battery it should be semi warm only.
Are you using stock servos? Digital servos can draw too much power, stock servos are analog.
Is your belt too tight? When you land and cut the throttle the head should spin for about 20-30 seconds. I keep my belt loose. I loosen it a bit and hold the tail rotor and turn the main rotor. When it skips a tooth then I tighten it a tiny bit and that is it. I also put graphite into the tube to lube the belt, that really helps (or other dry lubricant).
Check those things and let us know.