Aside from a dud, the lifespan should be more than the lifespan on smartphones and computers as the R/W demands are small. The capacitors in the device will die before wear becomes a problem. And if you consider the lack of moving parts it even has a chance to outlast a traditional hard drive.
Modern flash drives that replace hard drives have controllers that spread wear evenly and even have reserved space to cover a small amount of early failure. Simpler flash devices like microSD cards don't have these features and so are not suited for heavy use.