How much water to drink while exercising?

"If you must exercise, drink two to four glasses of cool, nonalcoholic fluids each hour."
Here's the rest from the CDC.
It's been in the 90s here, with heat index into the 100s. So here's what I did--yes, I'm crazy; let's establish that up front--I went riding from about 3-4:30 p.m. Heat of the day, blah blah blah.
I'm accustomed to exercising and I'm in and out of the office all day. But as I found out, I am not accustomed to exercising from 3-4:30 p.m. in early August.
Four bottles of water and one Gatorade.
Did OK but I realized about halfway through that I needed to be halfway through. Brutal. I rested in the shade several times. Doused my head in water in a bathroom sink.
Careful out there, Ye Fellow Residents of the Deep South.