On the 3rd or 4th node. I just did a NL/BB after the first node due to the fact that the 2nd node was growing deformed. So they say 3 or 4 but honestly i thinik you can do it whenever.
i experimented with one of my plants by cuting the second set of leaves and it looked weird for 2 weeks then it branched and looked like 2 plants it made a y and looked awesome until it got root rot from res temp being too high and the root rot kept jackin with my ph i used h2o2 and got rid of the bad parts of the roots and added air pumps to the res and added superthrive and some root stimulator to get it to grow new roots but it died because i caught it too late