After (or during) an hour or three of hard, sweaty running and/or biking, how much salt do I need beyond my normal diet?
It really depends but you need other electrolytes beyond just sodium, more specifically potassium and magnesium. A serving or two of a electrolytes beverage like Ultima replenisher should be sufficient. Of course this greatly depends on the specific workout session so use your own judgement and definitely do more specific research. You also may need to supplement your exercise with carbs so that you replenish your glycogen levels.
Measure all ingredients precisely. Small variations can make the drink less effective or even harmful. Mix the following:
Probably want to add more sugar if you are doing long intense exercise, unless you already use something else to replenish your glycogen levels.
Here is an excellent article on electrolytes and bicycling. If you sweat a liter per hour, a reasonable amount, a typical loss per hour is 1,300g for sodium (3.25g of table salt), and 230g for potassium. This is in the neighborhood of Mike's recipe.
For an estimate on how much you sweat, you can subtract your post-exercise weight from your pre-exercise weight and add the amount of liquids you drank in between.
Of course, everybody is different and "your performance may vary."