So I know the real answer is "it depends on your body and metabolism", but I thought I would put this question out to gather some general advice as a place to start.
When I did Eagleman last weekend, I knew the weather would be warm and that I would need to stay hydrated. I'm one of those lucky ones who can manage not to partake of the constantly available "water" during the swim leg, so I get all my hydration while on land. I didn't drink anything in T1 and began sipping while on the bike. For the entirety of the 56 mile ride, I downed a full 28oz water bottle (very weak PowerAde) and about 1/2 to 2/3 of another bottle (full strength PowerAde). I tossed my empty bottle at one of the aid stations on the course (the one around Mile 42ish) and grabbed a replacement water bottle. I drank about 1/2 to 2/3 of that. Coming into T2, I felt fine. I didn't feel dehydrated, but still was a tiny bit thirsty.
Going out onto the run, my legs felt fine, but about a mile into the run, I noticed the sloshing around in my stomach of tons of liquid. It was such that I could actually hear the sloshing. Apparently I drank TOO much while on the bike. After that, my run was basically done even though my legs were begging me to run. It became a walk/run where the run lasted as long as I could handle the sloshing. Many times I contemplated just making myself *ahem* get rid of the excess liquid *ahem* but was more afraid of the immediate dehydration.
So my question then, is how much liquid is too much hydration? I know it's personal to each individual, but are there any schools of thought on this? Virtually all the articles I find online are about not drinking enough on the bike portion. I've got the hydration/nutrition down for the Olympic distance races, it's the Half Ironman I'm still figuring out.