As far as I know, these SSDs compute their own remaining lifetime indicator, which can be requested with S.M.A.R.T. commands. The number is usually or in most part derived from the number of write transactions, so, flash memory wear. Of course, every flash type/generation has it's own wearing characteristic, and the drive firmware needs to know about this in order to compute a useful number. These are usually worst case/statistic minimal numbers, I think I have read a test some time ago where a computer magazine has continually written to a range of SSD types, and most outperformed their predicted life time and internal computation/write cycles.
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