How long does the iPhone 5 battery last using a bluetooth qwerty keyboard?

I would like to buy an iPhone 5, but i remain a touchscreen only refusenik. Thus I will only get an iPhone if I can use it with an aftermarket keyboard add on. That means that it has to be able to run Bluetooth all the time, because I do not want to have to go into settings to switch it on everytime I want to use the thing, I want it to be as if Apple had had the foresight to make an iPhone option with a slide keyboard... :)

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