How long does it take for a iphone 5s to die with rhenflash light on?

It's storming the powers out and it's 4 in the morning and I want to no this for next time! I'm only 12:P

Impossible to say as it depends on other factors such as having Wi-Fi, Bluetooth or 3G enabled, if you use other apps as well, what the current battery percentage is and what state your battery in.

Any way, I wouldn't advice using it for a long time, the LED in it isn't designed to be on for longer than a few minutes. Any longer and it may burn out.