If it's a BlackBerry, that shouldn't be an issue. Turn off the mobile network and wifi and bluetooth. This will allow you to use the GPS function without burning up your battery as your phone searches for cell coverage. Now, you will have to make sure the maps are downloaded to your phone before you leave... I use wifi for this since it's far faster than the 2G/EDGE network where I live.

When I was out last year with my brother and dad, I used the TopoSports app (I have a BB Curve 3G). I like it because it uses the USGS 7.5 minute quads. I have a great paper topo of the area we were in, and I was familiar with it, having been there previously, but I used the app on my phone to tell me precisely where we were. This was helpful because shooting a bearing on a landmark can be tricky in deep forest like that. I wouldn't trust it without the map, but that's because I've been trained to use the map.

I find that if you turn off the radio, power it all the way down if it's going to be more than several hours before you need it again (such as overnight), put it to sleep between uses, and close out the app when putting it to sleep (so it doesn't run in the background), then I get pretty good battery life. I'd have to re-test, but we went two days on that trip and I'd only used about 20% of my battery life.

Also, it depends on your battery. An old battery has shorter life than a new one, and a lot of that has to do with how you charge it. Some rechargables need to be recharged often, even if they're not drained. Others need to be drained all the way before recharging.... and this includes the initial charge when you purchase the battery. I had a cell phone once that I charged before use just like the paper in the box told me to do. It wouldn't keep a charge for more than half a day, and this is brand-spanking-new. So I called AT&T about it, and the tech support rep said that that battery needed to be drained all the way before charging. I told her I'd done what the paper in the box said, and she laughed and said, "Yeah, the paper is wrong. I'll make a note of it and send it along." She sent me a new battery, too. I did what she said, and drained it all out before charging it up and by golly it worked. So check the requirements of your battery (Lithium, NiCad, etc.) to see how to maximize battery life.