Anyone know the exact conversion? Say for example, how much fatigue per second are you getting with 100 endurance?

The amount of fatigue you can endure until you collapse is based on endurance, willpower, agility and strength. If you are running or swimming or fighting, then your endurance is not regenerating. My characters with max attributes have a Constant Effect item with "Restore 9 points of fatigue per second on self" and they never tire.