Since the Mangekyou Sharingan loses its light over time, one might wonder how Obito can use his Mangekyou Sharingan repeatedly yet suffer no vision loss. The answer is Hashirama cells. Obito has a prosthetic arm made of Hashirama cells, which likely has healing regenerative properties that protect him from losing his eyesight. Thus Hashirama's cells had an effect almost like the Eternal Mangekyou Sharingan, as they stopped the blindness caused by the Mangekyou Sharingan, which meant Obito could spam Kamui without the negative side effect of losing his vision.
