I usually pratice this long pot for 120 balls. 2sets of reds(15x2x4) for every corner. The % is about 60-70%. My best result was to pot 13 reds consecutive.
If you cueing is straight and good. I think you can pot almost 80-90%. That's why I am still training myself.
I think the professinal players can pot 95 out of 100. If you ask him to pot 10. I think most of them can pot all ten.