They don't last forever but they do last longer than traditional strings. My experience with Elixirs, both on bass and acoustic guitar, is that they deaden over time but so subtily that you really don't notice how dead they are since your ear is continually hearing the same tone every time. Kind of like a someone you live with. They'll change over time but you really don't know how drastically until you look at a picture of them 5 years ago. Then you're like WOW, they've really changed. I get that feeling with Elixirs. I listened to the tone over and over again but really don't know how dead they are until I stick on a new set. Then I'm like WOW, those strings were dead.
One thing that will kill Elixirs faster is the use of a pick. The pick will scrape the coating off the string and expose the winds to the air and dirt. Really, you can make any string last longer simply by playing your bass only with clean hands, storing your bass in a case at room temerature, and every once in a while taking a cotton ball and rubbing your strings down with denatured alcohol. Some people even take them off and soak them in the stuff.
