I work 12 hour shifts, without the opportunity to charge. Plus I usually have errands before or after work. So, generally speaking, I will go about 14-15 hours with my phone off the charger. However, I usually still have at least a 40 or 50% charge remaining. To do this, I generally keep the screen at it's darkest setting and try not to check the phone constantly. I do what I need to do on the phone about once an hour, short of answering text messages. I also don't play games, have the sound on, or use a lot of memory-intensive apps, which I know helps substaintially.
However, the best I've done with fairly heavy usage was one VERY full day. Left my house at 5:00am, drove the airport, flew to Nashville, spent the day around town, flew home to Orlando, drove back from the airport, and made it to bed by around 1:00am the following morning. I used my phone for music, Facebook, and so on during the plane rides to and from, and during the bus rides around Nashville. Called a few people while I was at the airports, used Google Maps, GPS, and Yelp to help me navigate downtown Nashville and find some cool places to eat/drink, and used it as a timepiece since I was an idiot and left my watch on my night stand. Granted, I still left the screen off whenever possible, and when it was on, left it as dark as possible. I also turned off GPS whenever I didn't need it and left WiFi off the entire time. But I managed to get through a 20-hour day...with 5% battery life remaining.