Until this year, I had already been using a Waterpik toothbrush. This also had a NiMH battery. These are generally very sensitive to overcharging and deep discharging. With my recipe of not recharging the brush until it was almost empty, I was able to use it for about 7 years. This year I also bought this new toothbrush. I use it in the same way. I noticed that after a short time (approx. 4-5 uses), only one charging LED is still lit. However, I can still brush my teeth perfectly for at least 1 week.
Unfortunately, I will only be able to say more in a few years...

The service life is around 3 years, then a replacement battery can be used for a further 3 years

I have had this toothbrush for 2 x 2 years now, and always after approx. 2 years the battery gives up and is not even enough for 1 x tooth brushing (2 minutes). I was able to have the first one replaced under warranty, and the warranty device now has to be replaced again.

This device is definitely top and I would recommend it to anyone, but the battery life is very disappointing in my opinion. (Devices like this definitely need to survive more than 4 years).

I changed the batteries myself. Now the brush is better than when it was new.

I've been using the toothbrush daily for 2 years and the battery still lasts quite a long time.... (no idea how it compares to the first week, but it's easily enough...)