Mike, only speaking for myself and probably doing a disservice to the ORS thread, I am not a medical professional or scientist. I'm a simple guy & endurance athlete. The rule of thumb, (heck it's probably science based) is to stick with water for the first hour of endurance activity, if longer than that switch to something that puts electrolytes back into your system AND water. Many of us also supplement with an electrolyte tablet, like endurolytes or sportslegs, when going for multi hour runs and rides.

When you're done with a run or ride over 60 minutes, immediately replenish with coldest beer possible.