kakashi's sharingan in his eye is always on, I don't think it takes up any chakra, but he covers it for a few reasons.
#1 If he has it uncovered, he has to use it because he can't control it properly.
#2 He doesn't want too many ppl to know about his eye if they don't already know and
#3 He looks damn cool with that protector over his face [img]i/expressions/face-icon-small-smile.gif[/img]
We Haven't seen Itachi without his Sharingan on. I'm willing to bet that once the third comma in the eye comes, the Sharingan are permanent and can't be changed back(another reason why Kakashi can't make his eye normal) and doesn't use any Chakra to just have them on anymore.
Sasuke's obviously just haven't developed enough yet.
meh, who knows, i could be wrong, then again, we all could be wrong