I give the minimum dose daily as all my horses stay outside & are in this extreme heat of over 100. They are sweaty & crusted with salt at the end of the day. They do have shade & run in sheds but will sweat even in the shade this time of year. Most people I know give electrolytes daily down here.
Our horses are on them from the time it starts getting warm and sometimes get half a dose when they are working hard in the winter. That said I live in the muggy muggy south.. you can be covered head to toe in thick sweat just standing out at the barn not doing anything. My barn has automatic waterers... which I don't really like because you can't monitor their water intake, but I usually get him to drink a few sips after every ride from a bucket by the wash rack.
I use apple-a-day by finish line. My horse is in my care, which makes it easy to add electrolytes only when they are needed.
Even though H is a great drinker, he doesn't like his buffet of mineral and salt licks. I supplement with a half dose any time he's gross enough he needs to be hosed.
I generally only use electrolytes a couple days before, during, and right after competing in an endurance ride. Occasionally, I will elyte if I am doing a long (few hours or more) and fast conditioning ride on a really hot and muggy day. Shorter rides, arena work, and just standing around -- no electrolytes. Horses have loose salt available to them at all times but I will add a tbsp to their rations 3-4 times a week if I feel that they are not taking enough in.