How long is a water sample good for? Say I take a sample in a drinking water bottle and cap it, and drive 20 minutes or so for the test. Is it accurate after a certain time, or does it lose something as it sits?
20 minutes isn't that long. I wouldn't expect any problems. The chlorine level will go down over time but if you completly fill the bottle and keep it closed it shouldn't go down significantly in 20 minutes. It is also a good idea to keep it out of direct sunlight.
It should be okay. An opaque bottle would be better, I think. If you have a Leslie's (and perhaps other pool stores) they will give you a sample bottle to use. On the Leslie's bottle it states that it is best to have the water tested within an hour of collecting the sample.