I put them on Fella 24/7 but when he gets bored of looking like a ninja he hooks it up to a fence post and takes it off himself...
Nicolaread - horses can see through them in the dark - a horse I know only has one eye and he HAS to wear it 24/7 to protect his remaining eye - this was suggested by the vet and i'm sure he wouldn't recommend it if it would endanger the horse with the possibility of them walking into things.
A bit of both really. Mine has to wear one in all weathers except rain as he has an eye condition (ERU). When his eye flares up (like it has at the moment) or i know i can't get down in the morning (like tomorrow because i have an exam) it will stay on over night.
Im hoping he doesnt destroy the new one that came through today though like he did with his old one (although this one is a much better fit and is more sturdy!).