Here are the exact numbers because we have tested that device specifically.
We charged the Lumia 900 100% and then ran the rangefinder on full blast, high brightness, no bluetooth with the lock screen disabled. The result... 3 hours and 20 minutes. That's basically "fire hose" mode, which is worst case scenario. On the golf course your phone will lock after a few minutes, which means the GPS won't be running 100% of the time (in between shots).
The new version of the app has a power saver theme which uses a black background. The result there was 4.5 hours at full blast "fire hose" mode without locking the screen.
So... the bottom line... The Lumia 900 should EASILY make it through 36 holes of normal play without any issues.
It would be great to hear how you were using the app specifically. I definitely want to make sure there aren't any issues or circumstances we haven't accounted for. Just email support@theCaddie.com and we will take immediate action.
Founder & Application Architect