I have loads of flameless candles. I've put Duracell in one candle and Energizer in an identical one at the same time, and the Energizer one would run two weeks longer than the Duracell one on a 5 hour timer. That's a 70 hour difference. Imagine if I tested the Energizer batteries against generic, dollar store ones.
Vendors that do list battery life probably do their tests with many grades of batteries, but ultimately list the battery life based on the results of using premium ones. That would make the most sense in order to make the product look appealing. If they list the battery life as 500 hours on a 5 hour timer, and a customer uses cheap, dolllar store batteries, it's that customer who has the opportunity to tarnish the product with a negative review. So, many vendors probably aren't listing battery life because the number of hours varies depending upon how the customer treats the product. It's like buying two pairs of sneakers. One pair can go to your 90 year old father to wear around the house, and the other can go to your 21 year old son, who is a long distance runner. Both pairs aren't going to function the same after 6 months. Like any products, flameless candles are only going to function as well as you treat them.