Estimates only. Battery life is impacted by a number of factors that make predicting life difficult. Some DM devices feature a Battery Meter for far more accurate real world measurements and predictions!
At 1 position or fewer per 4 hours, GPS Time is a cold start, typically 30 to 120 seconds. At 1 position or more per hour, it's typically a hot start, typically 5 to 20 seconds (shorter time between fixes = quicker fix).