At home my horses have access to salt blocks 24/7, and they use them every day.  Their feed is 99% forage, with a very little bit of forage-based concentrate (no sugars, almost no carbohydrate), and a great vitamin/mineral pre-mix called BioEquine, so they regain a certain amount of electrolyte/salt from their diets.

However, when we're travelling, I have to supplement them with electrolytes, depending on the temperatures, humidity, and workload.  Last summer we drove to Kentucky with one of our horses for the NAJYRC.  The drive and the competition were completed over a 3 week period, in record-breaking temperatures.  The hay at the Horse Park was appalling, and while we'd brought a lot, we had to purchase hay at the venue, and our horse, accustomed to really great hay, wouldn't eat it.

I used a 1:1 home-made electrolyte made from table salt and lite salt.  I started the horse on that electrolyte combination added to a very wet and very large (5 gallons) alfalafa cube mash 2X/day 3 weeks prior to our departure.  He liked it, ate it well, and drank well.  I fed between 1 and 2 tbsp. (in two doses) per day, depending on temperature, humidity, workload, or hours on the trailer in extreme heat, and our horse weathered (pardon the pun) the whole trip and competition in great shape.