Obito Uchiha was one of the strongest Uchihas in history and downright one of the strongest characters in the narutoverse but something else made him unique. As an Uchiha, he had the Sharingan for his dojutsu and he eventually awakened the Mangekyo Sharingan. His Mangekyo Sharingan ability was called Kamui which was a teleportation jutsu which lets him teleport part or his whole body to his own dimension giving him the look of intangibility. Obito Uchiha used his Mangekyo Sharingan for close to 2 decades and it showed no sign of losing its light. Obito was unique in that way his eyes never lost its light no did it give him any fatigue from overusing it the way it affected Itachi and Sasuke when they used their Mangekyo Sharingan (For sasuke, it was before he got the Eternal Mangekyo Sharingan). Why was this so?
There are a few theories and here is one i strongly believe to be the case. Obito when found half-dead was treated by Madara and he(Madara) grafted Hashirama Senju’s flesh onto Obito’s even making him a prosthetic arm from the flesh and Hashirama’s chakra and DNA has a healing effect to it. It could have extended to his eyes and kept on healing them as they were being damage through use. A testament to that is Shisui’s eyes used by Danzo, he had a prosthetic arm made from Hashirama’s DNA to give him the prerequisite chakra level to use Shisui’s eye and probably to prevent it from fading into darkness.
Obito was also helped by tobi, a clone of white zetsu who were made from Hashirama’s cells and he cover obito’s body during the early stages of his awakening the Mangekyo Sharingan and the vitality of those cells must have seeped into obito mixing his Hashirama’s chakra with obito’s chakra giving him the ability to heal his eyes as they were being used.
This is my theory, would be looking forward to hear some of yours.