Non-rechargeable ones. I bought a load off AAAs last winter for my rear lights and didn’t use that many so how long will they last in the packet before I need to replace?. I always have a few spare in my pocket on a night ride and want to make sure they will work for a while if needed.

There is usually a use by date on them

Non-rechargeable ones least ages! Last winters will certainly be fine. I wouldn’t worry ’till about 3 years after you bought them.

They may last a lot less time if you take them out and expose them to major cold (think sub-zero) so either test them intermittently or keep them inside your clothing to minimise the effect of cold

If in the packet they will last years, but see the date on packet for guidance

Depends on the battery type. i.e. lead acid, lipo, li-on etc…
Google your battery type to find out more.