How long will iPhone 5 battery last?

I'm going to a concert tomorrow and I'm planning on my bring my phone with 100% there. With me taking pictures and videos, about how long will my battery last? I need after the concert because I'm getting picked up and it is extremely important I stay in contact with the person picking me up.

Ideas on how to assure that it doesn't die? And, how long will it last?

It's hard to say. I suggest you bring a portable power source

When its hits 20%, stop using it… 20% is plenty left to make sure you can still place calls etc
Video obviously uses alot of power as the screen is on all the time.