We are constantly working to improve this but here are the current stats:
iPhone 6 Plus:
7 hours +
iPhone 6:
6-7 hours
iPhone 5/5s:
5-6 hours
iPhone 4/4s:
4-5 hours
Tested in a chest pocket at -4
°C external temperature.
However, you can expect these times to be near halved on devices that are not new or with batteries in poor condition.
Furthermore, weather will influence the battery state. Try and keep your phone warm, perhaps in a chest pocket.