Originally Posted by Just Jeff
I guess a rule of thumb might be that as long as it still feels warm, rather than just "not cold," it's worth keeping in there.
I'm not sure about that. Your body is already producing energy to heat your core and extremities, and that heat is simply soaked up by the bottle. As long as the bottle isn't so cold that it causes your limbs to chill from contact, the flux in temp of a liquid filled bottle should be very low. Especially when inside the same insulation you are.
It would almost stand to reason that the more heated mass you have inside your insulation with you, the more stable the temp ought to be. Getting that mass warmed in the first place is where the energy is used.
Just a thought.