The surest way to test the battery usage by various services (GPS, screen, Wi-Fi, calls etc) is to insert a milliampmeter between one of the battery terminals and the phone, and record the current drawn when using those apps/services. I have done this by inserting a strip of two-sided adhesive tape with two very narrow strips of copper foil on the two surfaces; the strips are then connected through a recording multimeter or simple data logger. If in parallel you'll record the terminal voltage of the battery, you can easily calculate the power drawn during each service/program test (P=U*i).
As you can imagine, there will be an almost inifinte number of combinations and parameters to conside under varous use conditions, thus giving an average battery life may, or may not, be very useful. For example, the power used during a phone call will depend on the signal strength and sound level; the former, in turn, will depend on the distance to the trasmission power, atmospheric conditions, location etc etc.) Any number you'll see quoted around will easily have a +/-50 % error...
You use it non stop for a week, and then it just becomes a phone again.
