You should recharge your battery if the resting voltage (the voltage of the battery measured after a 30 second resting period) reaches down to 3.0 volts or if your regulated mod no longer receives enough volts from your battery for the mod to be able to keep up the vape performance you want from the mod... whichever occurs first. Most regulated mods cut off at a resting voltage of 3.2 volts. On a regulated mod, amps drawn from the battery by the mod are increased by the mod as the resting voltage of the battery declines. Most regulated mods are limited as for how many amps they are capable to draw from the battery. So, depending on the wattage you vape, the amp limit of the mod may be reached sooner than the aforementioned 3.2 volt cut-off, and, when this amp limit is reached, as a result the mod will automatically lower the power output─causing degraded vape performance.
Discharging a battery all the way down to 2.5 volts (resting voltage) only once or twice on accident does not cause a lot of extra wear and tear on the battery, but should still be avoided nevertheless. NEVER (!) let the resting voltage reach below 2.0 volts.
The resting voltage is not the same as the voltage your mod receives from your battery whilst you press the fire button. This difference in voltage is called voltage sag, and, how big this voltage sag will be will depend on a whole plethora of variables... for example, it will depend on how many amps are being drawn from the battery.