I'd guess mine last about a year, too. I wondered if with some addition or other to the water they might last longer. However I don't think I've ever seen the shells deteriorate--what does that look like? Mine just seem to die. The shells seem fine--just empty.
I think it's lack of food after a while as well. Once they have the tanks clean they don't get enough minerals and such in their diets. It's tricky though because I don't want to feed them persay so as to avoid feeding the unwanted snails as well.
