Question: I've run a number of races, and there are almost always a number of water stations. I don't usually stop for water on the short races, but I'm not sure how long of a race I should do that for - when should I start drinking water? How much and often should I drink it?

Answer: 
It depends on the race and the weather.
At a marathon i stop at almost every station and grab a glass of water and have sip. 
Its better to hydrate in small sips from time to time.

With a bit of practice you can even learn how to drink while running so you don't lose to much time at the stations.
It's really much simpler than this. The marathon doctors have been recommending the following for a few years now, with the risk of hyponatremia in mind:

> 
> If you're thirsty, drink.
> 
> 
> 

Citations:

* *Runner's World* magazine, 2005: ["How much should you drink during a marathon?"](http://www.runnersworld.com/drinks-hydration/how-much-should-you-drink-during-marathon) (references, in particular, Tim Noakes, author of the widely-respected *Lore of Running* and author of the IMMDA statement below)
* Georgetown University Medical Center, 2007: ["Runners: Let thirst be your guide"](http://explore.georgetown.edu/news/?ID=25347)
* British Journal of Sports Medicine, 2013: ["Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat"](http://bjsm.bmj.com/content/early/2013/09/20/bjsports-2013-092417.abstract)

Additionally, [this 2001 statement](http://www.usatf.org/groups/Coaches/library/2007/hydration/IMMDAAdvisoryStatement.pdf) from the International Marathon Medical Directors Association (IMMDA), which concludes:

> 
> "Accordingly perhaps the wisest advice that can be provided to athletes in marathon races is that they should drink *ad libitum* and aim for ingestion rates that never exceed about 800 ml per hour."
> 
> 
>