To women who already have abs (and perhaps a personal fitness trainer):...

[JennT]

...what exercises do you do? This question is directed at women who already have abs (ie. do regular exercises to maintain them), or to fitness instructors: what exercises can I do at home, in order to get abs? How many times a week should one work out their abs, and for how long? Lastly, how long does it take to get moderately visible abs?

Note 1: I have a very healthy diet, and already do lots of cardio
Note 2: I am a university student, and don't have enough money to hire a personal fitness trainer

Thank you in advance for all the suggestions!