During the summer, flies will probably be around most of the time, so you may as well use it every day. UVA and UVB rays go through the cloud, so don't be misled by a cloudy day!
But I think it usually depends on how sensitive your horse is to the sun? I.e. Does the horse have pink on the nose, and particularly pink/white skin? I'm no expert but don't forget horses have evolved to live outside, so I don't imagine they are suseptible to skin cancer? I'm sure the hair mostly protects the vulnerable skin.
Most healthy horses cope ok in the heat, and will seek shade, so as long as the horse has a bit of shelter, should be ok?