Like anything in music, there isn't really a finish line, or a point where you can say: "Ok I have mastered this, there is no more to accomplish". I think what you are looking for is to become competent with an individual scale to the point where you can musically use it in fitting contexts. I agree with the items that topo listed out as being useful things to address in order to gain a higher fluency in scales, and just wanted to add one that helped me leaps and bounds. Learn to play through a scale or mode in certain intervals, for example, ascending the major scale in thirds. This means you play the first note, then the third of that note, the second note, then the third of THAT note within the scale, and so on and so forth, ascending and descending. Do this with all modes of a scale, with a metronome, in all intervals, and you'll be shocked with how it helps you develop your lines.
Another very important thing that I think people often have trouble with is making the scale musical. If you can rip through a melodic minor scale all over the neck in all 12 keys with impeccable technique, that is great, and you may feel like you have "mastered" the melodic minor scale. Then your in the heat of the moment with other musicians and it's time for you to improvise, ripping through that scale isn't going to sound like much other than what it is: playing a scale. You want to be crafting lines, not exercises. So I guess what I am trying to say is in your study of scales, be sure to differentiate practicing the scale itself(which is of course important), and USING the scale to create music.