Two reasons, the first is they literally don't know any better but are pretending otherwise which is a lot of people on the internet in general. The second is that light intensity is different in everyone's tanks. You can hit a cyphastrea with a lot of blue light, so if you run a bluer spectrum you can keep it higher, but they're really touchy towards yellower light so in a whiter spectrum like in your tank you'd want it to be shaded.
Maybe a couple of weeks and maybe never, it really depends on your lighting and tank parameters. If you run a yellower spectrum overall you're going to have a hard time getting the colors you want out of it.