I did a similar 2 hr ride yesterday and drank maybe 16-20 oz during the ride. I was probably down about 3-4 lbs but didn't weigh myself beforehand so I don't have precise numbers. I know from experience I can go hard for a couple of hours with no water and not suffer any ill effects. I weigh around 165lbs and I think you can lose 3-4% of bodyweight without any impact on performance. You might even be a little faster if there are hills involved.