Exertion time vs heart rate or %LT(Lactate Threshold)? Are there any tables out there that show how long a person can exert themselves at a given heart rate or %LT (assuming adequate fueling)? 

events lasting up to(LT = 185): 

3 hours: 170+ 
4 hours: 165 
6 hours: 150 
??? 
IM: 130?
Source - Metabolic Factors Limiting Performance in Marathon Runners

(While the above paper is aimed at marathon runners, I'm not aware of any reason it can't be applied to other endurance sports such as long-distance cycling.)

Exercise is fuelled by a combination of fat (plasma free fatty acids plus muscle triglycerides) and carbohydrate (plasma glucose plus muscle glycogen). The proportions used vary depending on the intensity of the exercise.



Exercise at a particular intensity can be maintained until you run out of carbohydrate - "hitting the wall" or "bonking". Intensity is expressed as a % of VO2max, which can be estimated from heart rate.

In order to work out how much carbohydrate you have available, you need to estimate how much glycogen you can store in your leg muscles, how much glycogen you can store in your liver and how much carbohydrate you can take on as food (gels, sports drinks, etc) during exercise. The methods section of the paper goes into this in detail.

In order to apply this practically to cycling you should take measurements of the power you can produce at various intensities (heart rates). If you don't have the equipment to measure power, then measure speed on a flat course and estimate the power from that.

Having done all that you should be able to calculate the total energy required for a particular distance at a particular speed and what proportion of that energy will come from carbohydrate. You also have an estimate of how much carbohydrate you have available and that will tell you if you're likely to hit the wall / bonk.