Is there a way to estimate the lifetime of a SD card? If not, is there a utility I can use that monitors its integrity and reliability so I know when to use a new SD card?
There are several tools that will check integrity of a filesystem, and recurring errors will point to a SD card reaching the end of its lifetime.
As for estimating the lifetime, the Sandisk Product Manual states:
However, I do not know how many read/writes the typical OS uses on average.