Going by what happened with Anakin in ROTS, my view of the Sith eyes is that it's a transformation that occurs when one allows themselves to be consumed with hate and all the traits of the Sith. Even in ROTJ, Palpatine was filled with evil thoughts of wiping out the Rebellion and removing Luke as a threat. He's the ultimate Sith, consumed entirely with the Dark Side once he lets his guard down in ROTS. At least that's how I take it because Anakin's in and out with his eyes in ROTS. As he's killing the Separatists, they go Sith on that close, then they're back to normal as he's crying with remorse and inner conflict over what he's doing, then when Obi-Wan "tuns on him" from his POV, he gets consumed and turns again, then his eyes are normal again as he's locked into Vader because he's in shock from everything and he first wakes wondering where Padme is, not really consumed with the Dark Side until he's told of her death, then I envision him always having the Sith eyes from that moment until he kills the Emperor, perhaps having moments where he reverts back to normal when he sould becomes conflicted with the light and the dark.