Originally Posted by whizzisat114

I know its a saft question but how long does it last?

Depends how big the bottle is and how big the NOS jets are - usually, not very long, that is why its only really suitable for drag racing
What bhp jets you run?
There are "dry" kits (NOS only) and "wet" kits (NOS and petrol into the inlet manifold) NOS makes the engine run lean, the bigger the NOS jets the leaner (its an oxegen supply system) more petrol is injected to balance this, the more petrol the richer it gets, its often set up rich to start (to prevent holed pistons and burnt exhaust valves) and gradually leaned off - as others have said the more NOS injected the shorter it lasts - running NOS can find weaknesses in the engine and trasmission
The wet nos kit can be used in conjunction with Methanol and water injection (again more suitable for drag racing)

Is it armed all the time?

Not normally (limited supply, remember)

Is it switched at full throttle or trigger?