It depends on your horse.
They don’t do much at night. If you’re concerned about bleaching, this is the option for you. But pretty much bleaching doesn’t affect the horse’s physiology, just the coat color. I live in NC and it can get quite hot during the day, so night time turnout was a way to avoid the flies and keep them in during the heat of the day. That barn had no shelters.
Half day, depending on what part of the day, turnout can encourage movement (grazing if there is grass) and interaction with peeps over the fenceline (if they share a common fenceline). Shelters are ideal to escape the sun.
I’ve done both. My mare totally treasured her stall at night to lay down and rest (raised out 24/7). There were no shelters in her turnout in Houston and was miserable with her fellow mares (she was lower on the totem pole and was miserable with 24/7 turnout with these mares). She moved to a smaller turnout with tree shade when I moved to stall boarding. She loved her stall and was soooooo much happier not being in a herd. My gelding wants to be out out out (raised half time in a stall) in his own 1 acre pasture with adjacent pasture buddies and a shared run-in with a gelding he likes to play with. He is a bona-fide weather wimp. HATES rain. If he didn’t have a nice shelter, he’d be miserable. I’ve ridden him when rain starts and he loses all focus and gets agitated. He can stand in a shelter all day but is less happy in a stall surrounded by buddies all day. I think in his case, he needs to see over distances to be happy. He can’t see distances in a barn, even though he was raised in one.
Carefully think about what will make your horse happiest. It might not be what you expect!