I'm leaving it a half day as by that time you've most likely been attended to by the medic and he's done his cough healing on you, which would be faster than if you left it to heal naturally, that in turn is the reason why the Medics will be part of the cohort.
We've never had an issue with it in Scotland, but I don't think we've fought all that far below freezing.
To be honest, I don't take the reports I read that seriously.
For one thing, a lot of UA larpers have had a serious bias against latex weapons with fibreglass cores, preferring "boffers" made with PVC pipes and soft foam. I think those people jump on any report of issues with fibreglass and blown them out of proportion/reality.
But also, after a long conversation with a US larper about fibreglass I realised they were using hollow fibreglass tubes with a large internal diameter, not solid fibreglass rods. I think they thought that seeing PVC pipes are hollow, fibreglass cores should naturally be hollow too. But who knows how hollow tubes compare to solid rods - they might be more sensitive to cold. The only time I've tried using hollow cardbon-fibre rods I found them extremely fragile in any temperature.
Not all solid fibreglass/carbon-fibre solid rods are the same either. They may have been using a more brittle type of rod and blamed a breakage on cold when it was actually just poor choice of material.
There are lots of fibreglass-rod latex weapons in the Nordic countries, and I don't think I've heard of problems in the cold.
I made some weapons like that about 20 years ago and they broke easily. I've never seen a solid rod break though.
Hollow tubes will always shatter after a while, as for low temps we were larping at -6 a short while back and no reports of anything like that.