There are 2 sides to this question. Firstly, is it the duration of how long your battery should last after a full charge. Secondly how long should the life of your vape battery last.
Most vape devices will have a battery that is designed to last for a moderate smoker for the day. The problem is we will all need a different level of puffs per day so it is hard to pin down exactly how a vaping device battery should last. There are also several different types of batteries you can use including single, double, different wattages and so many other variables.
So dependent on how much you vape and what your device is, and the type of battery depends on how long a standard charge will last. Please refer to the handbook that can with your device or search google.co.uk for further information on your specific device.
Once again, we find ourselves in a difficult situation to answer how long should a vaping battery device last before it completely dies. So many variables need to be considered including how much you use it, where it is stored, what type of battery it is, how often you charge it and even some of the simplest things like is it stored upright. A general rule of thumb is that your battery should last for round a year or more but again it is more than likely that checking your handbook should give you some sort of idea.