I have a 2.5 week old seedling that is only ~3 inches tall, but I just topped/fimmed the 5th node. I plan on starting LST, how long should I wait before tying them down? She is currently in a 3 gallon.
You can start LST at this point, but it is all preference. Personally, I would allow for a little more growth so that there is enough length on the main stem to bend without too much tension.
Some of my plants had a grow stop after topping due to high stress. I would wait and see how your plant performs, and if its doing well, i would suggest after 2-4 days from now on.