your mech should report low voltage when it gets to low, normally around 3.2. they generally have built in protection and wont fire once it gets down to that mark.
i normally change mine around 3.3 or when i can feel vape getting a little weak.
gerneally i'll get a days vape from around 8am until about 8-9pm then change batteries but then i all day vape cos im at home all day