Soon, I like to do it when there is enough roots to hold the 1 gal soil together but before they start to wrap. To soon and you have to be very careful to late and the plant thinks it needs to grow up rather that wider. I am starting to love the "smart pots", no roots to separate when transplanting, no shock, they breath, there wide.... I will never use plastic containers again. I highly recommend them. I was sceptical at first. I've head they have been around for a long time but had never seen them before. After using them, I now love them. No more plastic for me. Plastic works fine but there is a difference. try one side by side and look at the plant and then the roots when done.
I'm going to try a few outside this year. Another problem with plastic containers is they get hot as hell outside in the sun. Cooks the roots. Fine in the early spring when soil temps are needed but in summer they suck. Smart pots don't seem to get hot even early indoors before the shading of the leaves they stay cooler.
I know i need to get my other room built I have a 250w lumatek and a 6inch vortex just waiting for there new home which will become a mother room or in the more immediate future a Autoflower room for the white dwarfs!
Just get a pot ready with soil and test the height, when you like how it's going to sit in there, put your hand over the container with the plant between your fingers. Flip it over and tap on the bottom of the container, squeeze a bit if you have to. It should fall out into your hand. Flip it into it's new home and put more soil around and water well and then don't water for awhile. Don't sweat it if you break any roots MJ can take it. But the less stress the better.
