You'd be better off throwing some _________ Brand bottles of water in your car (fill in the blank with your brand of choice). IF the canteen was sterilized and IF the water was disinfected and IF you could get a good seal on the canteen then it could last indefinitely. Bacteria have to get into the canteen to make the water go bad. They don't have arms, legs, flippers, or wings so an air tight seal really isn't necessary (they can't crawl or fly or otherwise shimmy their way anywhere). Pasteur proved that (that an air tight seal isn't necessary to prevent contamination) with a goose neck flask in the 1800s. However, you are more likely to get mold growth than bacteria. Bacteria need a carbon source. There are varieties of mold that can fix CO2 out of the air and thus don't need a soluble carbon source.
That being said, I have double distilled deionized and 0.2 micron filtered water sitting on my lab bench in a 50 mL falcon tube (factory sterilized) that is opened daily and has failed grow anything yet. It has been sitting there for months.
And, what's comming out of your tap isn't much better (if any) than lake water anyway.