i have a futaba 7ch and i would like to know how long should i set the timer. the engine is an O.S. SX .30 on 15% fuel

I don't screw with a timer... I just do a fly-by every so often to see if I'm down to just the header tank or not... if I am, I know I've got just another couple of minutes, and I keep it low and close-by so I can bring it down quickly when it runs out.

mine doesnt have a header tank do you need one . if so how do i install one on a nexus 30

It's nice to have, but not necessary... it's supposed to keep air out of you're carb's fuel line and keep more consistant fuel flow to the carb. If you don't have a header tank, hopefully you can see your main tank's fuel level easily... just do a fly-by and see how low you're getting. You should be able to mount a header tank on the side of the heli chassis if you want to. If there isn't one made specifically for your heli, you could buy the Raptor header tank and mounting bracket, and drill you chassis and mount it wherever you think best. When my heli gets down to the header tank, it doesn't run as well, so I don't do anything too brave at that point... I usually keep it close and low until it runs out of nitro.

You have to fly it a few times and get an idea. The 9C has 3 timers on it. I have the first set to total transmitter time since charge, second to time on since I hit the switch and third is time on when throttle higher than a quarter. Course, you can't seen any of them when you are flyin so it really doesn't matter. I also just look at the tank once in a while. Flew without a header for years. I have one now though. Nice for aerobatics.

set the timer for 12-15 min max. if you add a header tank add 5 moe min.