I usually wait until mine have 4 or 5 nodes. You need enough spacing between nodes that you aren’t damaging growth tips when you start training, but if you wait too long the main stem will get too stiff. I Iet that one go until she started stretching in preflower, but she was still small and bendy.
This journal covers more intense techniques on photoperiod plants, but @Arrow has loads of good info and a lot of the techniques he uses can be used or mimicked with low stress training.
And this is my auto journal. I’m still learning too, but this site is great!