This is really a preference type thing and kind of open to your setup. If your using more powerful lights like a 600w or 1000w you can wait til later and let the plants get larger because your lights would penetrate deeper into the growth. For a 400w or smaller youll want to start sooner. Also it depends on how much vertical space you have. Remember that there is only a certain amount of space you will have to tie the plant down. You will be able to grow a larger plant if you can allow say 8" Growth tie down 8" tie again, etc than say if you only had 5" of growth tied 5" tied, etc. Make sense? I would say the absolute minimm would be 5 nodes before you tie down for the first time. Hope this helps. Any more Q's let me know!