Geasses seem to have multiple effects, powers and drawbacks.
It's like there's a border in what you can do and what power you have.
When you have a very efficient Geass, working with no direct eye contact and great range, the action seems to be short in time. On the contrary, if you have small range and direct eye contact, you can have long time effect.
Then there's the control the user has on the power. Maybe over time, by cautious use, you can extend the power and have some limitations removed. But if you use it too often, the geass gets you before you can control it.

From what we see of rollo's and the emperor, they may have attained a higher control ability than Lelouch, they can restrict the power to only what's needed for their immediate use, whereas Lelouch seems to use it without much control, leading him to problems when the Geass becomes uncontrollable.

On the other hand, it's strange CC doesn't help him control the Geass if it's possible, and "only" gives him a masking contact lens. If it's possible to get used to the geass and control its shape, power, time range, why wouldn't she help him that way?
It also seems that the Geass may affect one's reason, because just when Lelouch replied to CC he would control his geass, then it becomes uncontrollable. I have wait for more data on the use of the geass.