Garmin rates it at 14 hours. However, the first few IM posts with it have it dying at 10:15 - 10:30. One person posted a sub-9 with lots of capacity left. Not a whole lot of science, but also probably more of a real-life experience. At least one had GLONASS on and one off. So, I would rate 14 hours as a maybe, but I would not bank on more than 10.Interesting, I thought the 735XT could last more than 10.5 hours...at least from what reviews say it can stick around 14 hours of GPS use.
Given that Garmin just released the fēnix Chronos watches, I would expect the f3 line to continue a while longer than January. Plus, there is really nothing in the market even close to the fēnix 3, so Garmin can probably afford to sit on the current suite for a while longer while doing development on the next gen.The fenix 3 was launched on Jan 2015, with a refresh in Feb 2016 (HR and Sapphire options). I expect it to be renewed by Jan/Feb 2017 at most.