How much concentration do I need to practice vipassana? I have read some articles and book, it says that you need to develop some basic level of concentration before you can start practicing vipassana(insight) meditation.

What are the criteria to know if I already reached that basic level of concentration?

Thank you very much!
In Tibetan tradition, before you can do Vipashyana, you have to practice Shamatha-type meditation until you learn to attain so-called "Calm Abiding" or "Fully Pacified Stable One-Pointed Effortless Meditative Equipoise".

According to a traditional presentation, Calm Abiding is considered attained when a meditator can effortlessly focus on a chosen object of observation, without getting distracted and without falling into lethargy, for a substantial period of time. 


"substantial period" is  a subjective measure. Some say it is 2 minutes, some say 10 or 15.
"object of observation" means any object or theme, like a flame of a candle, flower, a far away mountain peak -- or a visualized/imaginary object of any kind, the most popular being an image of Buddha statue. (With imaginary objects it is best to use something very familiar that you remember very well.)
"without getting distracted" means without forgetting about meditation and getting carried away in thoughts.
"without falling into lethargy" means, without getting sleepy or even losing clarity of mind.
"effortlessly" means without fighting with oneself over distractions and lethargy.


Here is a set of more detailed guidelines on achieving Calm Abiding: an excerpt from a lecture given by Gelug meditation master Lati Rinpoche.