This article has a chart of average electrolyte losses in sweat.

Unfortunately it doesn't give a primary source reference, and the single position paper they cite recommends sodium intake of 500-700 mg per liter of water, not the higher amount the article recommends (and it doesn't discuss other electrolytes). I didn't go through and read all the studies cited in the ACSM position paper.

But even according to the ACSM's lower guidelines, on a ride of moderate distance, heat, humidity and intensity - say four hours sweating out two regular sized bottles of water (1.3 liters) an hour - that would translate into 2.5 - 3.5 grams of sodium needing to be replaced, or a teaspoon to a teaspoon and a half of salt.