I'm going into the wilderness hiking at the weekend for ~7 days. I'm taking my phone in case of emergency, but I'm also thinking about putting the phone in airplane mode and using it for taking photos. Let's for arguments sake say 20 photos per day. I doubt I'm going to be able to charge my iPhone while I'm out there, so I wondered if anyone has any experience using the phone like this, with regards to battery life on a single charge. how long is it likely to last?