What's the longest you have used a cellphone for guidance? I think I did it for a 100k. I didn't really like it. My phone screen is horrible in daylight though, I'm going to work on that with the next one. I found that it wasn't loud enough. I'm pretty sure in battery saver mode my garmin 830 would have lasted for all of PBP. I used a route that split the ride in 600k segments though, just to be safe since it's out and back.
Someone I have ridden with once used rwgps as a backup on a fleche, so 24 hours. He had his phone hooked up to a giant backup battery. I was trying to figure out if there was a woman talking to us before I realized he had his phone giving voice guidance in his back pocket.