How much battery life do I actually need?

For most runners doing sessions under two hours, anything in the 15–25 hour GPS range is plenty. Ultra runners and multi-day adventurers in the reviews we read specifically call out 30+ hour multi-band GPS modes (Forerunner 965, Pace 4 in lower-accuracy modes) as the threshold where the watch stops being a limiting factor. Daily smartwatch battery of 7+ days is the figure casual users tend to cite as the comfort zone.