Funny thing about Mr Tuffy's is that the company claims "One pair of liners will last a lifetime" but on many online store websites, you'll read in the product description: "Guaranteed for 1 year"...
Personally, I think it's like anything else and one has to do a periodic check, maybe once or twice a year, to inspect the liners. It's only a polyurethane strip and many "local environmental factors" could cause them to dry out and start cracking or fall apart. Could just consider/add tire liner inspection as part of your regular maintenance schedule.
There are a few commuters I know that prefer Panaracer's Flat-Away Tire Liners since they are adhesive backed and don't float around the tire...which they claim less wear on the inner tubes from the rubbing effects. But they change them once a year.
Personally, I use kevlar belted tires and just deal with the extra weight of thorn resistant tubes. At least for me, it's the best compromise for the conditions on my commute...others may want or need extra flat protection, whether it be tire liners and/or tire sealant with flat resistant tires like armadillo or...