I have no idea how long the batteries last. The ones I saw elsewhere use those batteries like you might use in a watch. They are little discs.

I would use them only if all the lights were off, like in a theater or in the car.

Funny you asked about these, I have a 16 year old who saw the larger ones and thought they'd be a great Xmas prresent to put on her list. When you use one of the 40% off coupons, the price isn't so steep for a novelty gift. If they use those expensive disk batteries then I probably won't replace them and just considerr them the novelty they were designed to be.

Now on the flip side, my 84 year old mother saw them and thought they were a terrific idea as she used to knit even in the dark movie theaters.

A friend just bought them from a LYS. She says they use hearing aid batteries so that the batteries will last longer. She had not tried them as yet.