Hi Sun,

Concerning the statement

there is no quantified period of time to which this statement applies. What the manufacturer is trying to communicate is that you should not design your circuit to operate at the limit of the ratings. If, for example, a particular chip has a Maximum Supply Voltage rating of 25V, don't use it with a supply of 25V, but with perhaps 20V instead. Yes, it would hold up to the 25 V supply and function quite normally, but for a significantly reduced period of time. Another way of stating the same thing would be to say, "Operation of this device at its Maximum Rated conditions will significantly reduce its MTBF (Mean Time Between Failure) rating."

Now, all of this becomes immediately and completely irrelevant if you put the chip into a circuit where it will be exposed to conditions beyond its, (What's the phrase? Wait for it.), Absolute Maximum (or minimum if you're talking about a negative voltage) condition. In such a case, the phrase you asked about simply doesn't apply.

Good Luck.