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I believe the change oil light comes on at about 15 percent remaining...can't remember the exact percentage, but it does not come on at zero life left.
I also believe you can read it via a Tech 2, but that does not help people who have not invested in this hand scanner.
This is my oil change schedule. I'm a firm believer in the GM oil life system. One reason, is I know some of the people who worked years on this project, and know the testing they did to confirm their algorithms. However, I will always perform my first oil change at approx. 1-2K miles, and from then on, let the computer tell me when to change the oil.
Only other times I would deviate is if I was running a foam air filter (which I will never do), and off-roading in dirt (especially sand), or if I was submerged in deep water. The computer cannot allow for extra dirt intake and foam filters are noted to do a poor job of filtering dirt when compared to a paper filter; therefore, the computer has no idea how much dirt is entering the engine.
The algorithms for the oil change light are based upon engine operating conditions at startup, how long the engine is run between startups, how hot it gets each drive cycle, and most importantly, the oil you use, and the fact that the engine is as well sealed as it was from the factory.
Oil grade is important. You must use oil that is specified by GM, this is what the algorithm is based upon. It knows how this oil will degrade over a period of time, when subjected to heat, cold, etc. Using a lower grade oil will not result in an accurate reading.
Using an oil that exceeds the specification such as a synthetic oil is ok.