Dave, an RO membrane really shouldn't get used up if properly maintained. It may, however, get clogged. The change you would expect is less production (more waste water) as opposed to increased TDS. Using a backflush kit regularly should preserve the membrane for a long time. I suppose that a clogged membrand could be damaged by the pressure and start to "leak", but I haven't seen that problem.
You might want to contact The Filter Guys via email...they are really helpful and a cheaper source for filters and replacement cartridges than any place I've seen. The specialize in just aquarium filtration (RO, DI, etc.) and are great to work with.