First time we see sasuke use is MS, maybe tsukuyomi or some sort of genjutsu.
again with the tsukuyomi or the strange genjutsu.
Sasuke using Amaterasu
and finally, sasuke taking off the amaterasu.
After only 4 usage of is MS he is already losing sight. This is definitely faster then Itachi, proof:
-I won't get the pic for this one, I think everyone as seen them more then enough already-
1: 1st known time he used MS was on Sasuke when he was young. Sasuke was put into the tsukuyomi genjutsu.
2: 2nd known usage was several years later on Kakashi with the tsukuyomi again. We have no idea on whether or not he used it during that time period but I have reason to believe he did, I will explain later.
3: 3rd know usage was a couple of weeks/days after as he use tsukuyomi on sasuke...again.
4: 4th time was on the same day as he used amaterasu to escape Jiraiya flesh prison.
Until is last fight with Sasuke I cannot recall if he used is MS again.
Unknown time: The latest spoilers pictures of manga number 463 have lead me to think that Itachi trained is MS for at least one time; he trained in the summoning of Susanoo. In the spoiler we can see sasuke protected by an incomplete Susanoo skeleton, which indicate that to complete it, you need a certain level of training.
Again this is only speculation, but I would be extremely surprise to learn that Itachi never trained is MS. I know what you are about to answer tough: Sasuke can possibly have trained is MS too, which is very likely. However the debate here is on the time period. And from what I have demonstrated, Sasuke as started to lose is sight less then days after is first MS usage V.S Itachi who is known to have used it for years before feeling it's drawback (first time we have seen it, is in is fight with sasuke. We can therefore assume he was already losing sight for a certain period before)and both have relatively used it in the same amount ( sasuke used it a bit more).
Sasuke's Sharingan is obviously deteriorating at a faster rate...