How much sodium you need on a long run is mostly determined by whether you're a salty sweater, and there's an easy + free way to check
The industry answer is take our product every 45 minutes which is not the right answer. The variable is sweat sodium concentration and it varies a lot between people (by something like a factor of four). some people lose 300mg per litre, some lose 1800.
Free test: do a hard hour in a dark shirt, let it dry, look for white crusting round the salt lines. crusty means high concentration. no crust means you probably need less than the packet tells you.
I'm crusty, so I take more than the label suggests, about half a teaspoon of celtic sea salt in a bottle. I went to that one because its unrefined and sun-dried rather than kiln-processed, so its still carrying the magnesium and potassium that came out of the seawater with it, which felt more sensible than sodium on its own for a four hour effort. My partner runs the same distances and takes almost nothing and is fine. same weather, same pace, completely different requirements.
The only thing I'd say is dont start experimenting with this on race day and if you're going long and drinking a lot of plain water, sodium is the thing that stops you getting into trouble, not the thing causing it.