I use my iPhone off and on all day long. I am often away from a desk to charge it from, so my battery will run out usually in around five hours.
The Mophie Juice Pack Air is a great looking (IMHO) case that also packs a 1700mAh battery that will fully recharge your iPhone 5/5s pretty quickly to get you through the rest of the day.
Mophie Juice Pack Air on Amazon.co.uk (affiliate link)