What is the expected battery life of an iPhone 4s?

If I only use it to play music during my journeys (one is 15 minutes and the other two are about an hour and a half), how long is my battery likely to last? I'm travelling by coach so I can't charge it, like I could on a train.

-to get best charge backup,
empty your battery or use fully to empty battery, then charge fully, don't use phone while charging,
-phone need lesser charge for music than call or vid chat,
- don't charge time to time without finishing charge, it reduce your battery life,
-also uninstall unwanted apps from phone,
- i use this methods after studying some topics in engineering and after surfing web,
then i get best battery life

Since your screen isn't on (it will go dormant) it will last alot longer.