Hi Agrelon, your body just tells you it's time to go back home. 4/5 hours overpowered is enough for a session. If you fix the cramp issue it will tell you another, maybe less friendly, way. If you need to sail more than five hours in a row, for instance in a long distance event, bring food and beverage with you and moreover try to be "just" well powered. Only a few heroes can keep up overpowered and hope to finish.
Personally I don't suffer cramps too much but rather a kind of fever with pain everywhere. Two such days and I don't like WS anymore... till the next weekend.