It has been perhaps 6 months on generic Chinese CR2450 batteries. I just invested in some Energizer batteries to see if it might be better.
As a side note, I doubt that I would choose these sensors for a fresh install. It was nice to re-use the ones that were already installed when we moved in, but there is nothing stellar about them. The temperature feature is curious, as it only tells me how much heat is being transferred to/from outside. The size and styling are subjective, but I find them to be a bit dated now.