Hey Carmen! I planted my autos in their final pots so I can't tell you when to transplant in terms of LST, but when to start bending is determined by the plant's height.
You want to be able to get the top lower than the lowest node you'll be keeping, and that's tough to do with a short plant. Most folks who try that end up with a plant at an upward angle but never get that bend that makes LST work. Leaning over a plant doesn't do anything to move the growth hormones around.
You can see in
this post that by the third time bending it down the top is below the lowest node. It takes a few days to get it there to make sure nothing snaps.