Hello everyone! I'm a new member (and hope to go ahead with my membership)

I need an advice for a workout.
I'm a student, and so I don't have enough time to work my body out. I know this is not a good way to build a sixpack but... Is there any way to get it?
I mean, do I get it if I train, for example, twice a week? Or even three times a week...
I'm quite tall and thin (I'm gonna add my pictures as soon as I come back home), I have a really awful "four-pack" (awful because it is not absolutely ripped), and really low bodyfat...
Any advice? Is it ok to train my abs three times a week? If so, for how long do I have to work out? Which exercises fit best my needs? Thank you in advance and sorry for my poor english!