Do MacBook Air's SSDs fail?
MacBook Airs ship with either Toshiba or Samsung MLC SSD drives.
No SSD drives are fail-safe.
Lifespan of an SSD drive depends on what wear levelling techniques are employed by the manufacturer and the operating system and what exactly a user does with his/her computer. There is no reliable figure.
My own view with SSDs (as it has been with hard drives) is prepare for the worst and backup regularly. Take advantage of the manufacturer's warranty should a drive fail.