The light never goes away with EMS but if you use izanagi or izanami it will shut of one eye forever.
Its like ur eyes watching tv day and night it would slowly become weak and blind you, if you watch it less often your eyes become weaker slowly. When you get ems no matter how many tv you watch day n night your eyes don't go weak but if you use izanagi its like a sharp blade pokes right into ur eye you still go GG.
But the question here is whether Itachi's eyes will heal because hes Edo now and we know Edo regenerates. Or even if he gets resummoned will both his eyes be back since Edos get summoned as they were before they die.