I have had some already for 2 years or so that are still going strong (in 4 or 5 tanks). From what I have read, their lifespan is 2-6 years depending on species, though its almost impossible to gauge their age upon collection as size can be dependent on so many factors. Of course temp, diet, and parameters can really effect their lifespan becacuse it effects their growth and metabolism.
An important thing to remember is that though these are relatively hardy snails, they are still more sensitive than alot of fish. They don't tolerate shifts in parameters or less than ideal situations well. Often what will work for most fish (especially higher nitrate readings over 20) will be really detrimental to snails.
Its also important to recognize that these guys are in transit for quite awhile before they arrive at people like me who distribute them. Its part of the reason why I won't send them out until they have been in my tanks for a week or two, this way i can ensure that they have gotten a really good meal and are not starving for another leg to their journey.
Also, its important to really add these snails to a well established tank. In my experience, tey don't really readily take foods (prepared or zucchini, etc) until they are practically starving. Typcally they will ignore those foods for the biofilm algaes which they prefer. Its another reason why I recommend 1 per 10g, 5g max in stocking and only to a tank with some existing algae. They, like any snail, can become very weakened and struggle to feed if there is not a ready food source that they like.
I don't know if this was the case at all for the OP, but these are my observations and experiences in my tanks when dealing with these particular snails. Hope that helps.