(Proof) How the Mangekyou Blindness works.
Itachi had the MS for a long, long time and he went almost completely blind only during his fight with Sasuke.
Meanwhile, Sasuke had the MS for only a few days and he went almost completely blind in a really short time.
And then there are Kakashi and Tobi.
Kakashi has been using the MS only since the start of Part II, but he has been relying on Kamui really a lot, especially in the recent fights, but he has no signs of blindness.
Tobi is probably the person that used the MS the most, since his whole fightning style revolves around Kamui's defense and during a fight he has it active for pretty much the entire fight.
So why such a difference? Why Sasuke went blind so fast and Itachi could keep his MS much longer? Why Kakashi and Tobi have got no effect?
To go blind you must use MS techniques. Every technique used consumes a little of the MS light.
Each technique puts a different strain on the eyes, based on how much chakra they use and how much strain it puts on the body. So every technique causes the light in the eyes to deteriorate at a different rate.
And to me the most straining MS tech is Susanoo.
The reason that i believe this, is because:
While Amaterasu, Tsukuyomi, Kakashi's Kamui and Kotoamatsukami are oneshot jutsu (and as such, you shoot them at your target, it causes immediate strain but then your eye has time to rest), Susanoo is a costantly active technique, which is used to protect the user.
And since Susanoo turned on is always active, it continuely puts a strain on the eyes, and makes the user feel pain in their body, like Sasuke said.
So it must mean that the reason that Itachi was able to keep his light for much longer, is because he only used Amaterasu and Tsukuyomi when it was necessary, and didn't really use Susanoo that much.
Meanwhile, Sasuke abused every MS technique in his fights, especially Susanoo which was costantly active, putting an enormous strain in his body (as demonstrated by the fact that Karin had to heal him pretty much the entire time)
The reason why Kakashi still has got plenty of light left is because he didn't really use Kamui that much until his fights in the war, and because it is a oneshot jutsu, it only puts a minimum strain on the eyes.
And since Kakashi has no Susanoo and only one eye, he will not lose the light very soon.
For Tobi my guess is because Kamui is a very light MS tech, and it doesn't really put that much strain since the only thing it does is transport parts of your body to the other dimension. Couple with the Senju DNA he has and he probably still has a buttload of light left on his eye.
But what about EMS?
Since EMS doesn't lose light, it actually allows Sasuke to keep Susanoo active without feeling any pain to his body. Same for Amaterasu and Blaze Release.
Remember that the eyes are connected to the brain, losing light is very stressful to the brain and will cause a lot of pain to the body.
And since Chakra is connected to lifeforce, it makes sense that MS techs, which cause a lot of stress, would use a lot of it. So it also makes sense if EMS would use less Chakra than MS.
Afterall, wouldn't it be much harder to fight while your eyes are burning like hell?