My general guideline for caffeine in endurance situations was taken from my coaching mentor, who said "Don't use it until you need the angels to start singing".
I use strictly uncaffeinated TW, either the Naked or Mandarin formulas (Generally mandarin on the bike, and naked on the run, because usually I'm sick of flavor by the run, haha). In general, on the bike I use TW anywhere from 1.5-2x concentration, depending on weather conditions, intensity of work, etc. Has she done a sweat test, even a rudimentary one where she weighs herself before and after a workout, as well as all the fluids she's taken in? That's a good start to determine how much per hour/etc.
In general during the summer, I will take in a 24oz bottle on the bike once every 1:15 or 1:30, taking 3-4oz sips every 15 minutes. That's kind of the key with TW, is that you use it to top off the tank, not to completely refill it all in one go. If I'm running after the ride, about halfway through my ride I'll start taking in more so I go through a 24oz bottle once an hour to get a little more in the tank for the run, since it's harder to process anything on the run than on the bike.
As for carrying that much fuel for a longass ride in a race, I'll use a "sludge bottle" of highly concentrated TW, marking off graduated lines with # of calories that I've measured out, so I know exactly how much to mix in my BTA hydration tank with water from an aid station.
Level II USAT Coach | Level 3 USAC Coach | NASM-CPT
Team Zoot | Tailwind Trailblazer
I can tell you why you're sick, I just can't write you an Rx