How long will it take them to do this, and how is it done? Is it actually physically/electronically done to a large number of chips, or is there some sort or thoeretical extrapolation done?
Yes to both. Experiments are performed on a sample of units large enough to product data that is statistically significant. No one can absolutely guarantee any individual device (except to the extent that they can promise to give you your money back if it fails). What they can do is demonstrate to a customer that a sample base of x units showed no more that y failures under the conditions of the experiment. That gives a confidence level that the customer can rely on and predict.
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