Makers will disagree and part of that is because all batteries are not the same. Icom says to fully charge their lithium battery packs every 3 months, IIRC. Other makers say they can go a year between charges, others six months. Good NiMh cells from Panasonic and Eneloop may hold a 75% charge after a year.
But without knowing what you've got...Buy a $20 digital multimeter. Don't assume it is accurate, because many are not. Charge your spare battery, let it sit overnight, then measure the voltage. Now put it away for two or three months, measure again. Whatever your personal tolerance is, that's when recharge time is. If you can accept a 75% charged "emergency" battery...you can let it discharge that much. If you think 85% is all you want to let it go, pick that point.
If your battery pack is maybe rated at 7.2 volts fully charged? That would be two lithium cells in the pack, nominally 3.6 volts each. Maybe they charge to 4+ volts...again that varies. See
Charging Lithium-Ion Batteries
for some notes on how charging the batteries affects their overall life, and what voltage (per cell, and yours probably have two) really reflects what charge capacity.
A real emergency backup would be the battery tray, plus some "10 year shelf life" alkalines, easily replaced at exorbitant convenience store prices if there's a real emergency.
And FWIW, think about keeping the spare batteries (actually, anything with a lithium battery) in something like a metal surplus ammo can. Lithium has this funny thing about spontaneously bursting into flames. Metal containers can make that an inconvenience, instead of a problem.