When riding outdoors, I sweat a little bit. I mean, I can be out in 95°F weather with 90% relative humidity and I may get a drip to trickle from my temple. At a stop, in the sun, I may get a few drips down my back. So, my rule of thumb for an outdoor ride is one 24oz bottle of water per hour, and I may only lose 1 lb per hour at most.

Indoor training is another story. I set the AC to 68°F, have a fan blasting 25 mph wind onto my entire body, a dehumidifier keeping the relative humidity below 50%, and this is what my floor typically looks like after a 2 hr training session...

Sweat by silversx80, on Flickr

What's more is that even drinking 2 quarts of water per hour, I still will lose 5-7 lbs in a 2-hour session.