Updates: Lights are on about 45% at 45cm from the plant. I watered 2L of water once during the week. I added a layer of fresh worm castings from my worm bin directly to the top of the soil. I also added some more biobizz light mix to cover the castings and fill up the pot. I put a layer of rice husks on top as a mulch layer. I didn't add more water right away so I think that after the next watering it will leach into the soil more.
The Good News: Overall, things are looking good so far. The 3rd node is coming in on all plants. The one that sprouted last is still a little smaller. They have grown vertically only a little, but the new nodes are coming in.
The Bad News: 2 of the plants are showing discolorations on their leaves. I see what I think are signs of thrips on the leaves. Apart from the discoloration I notice what look like tiny snail trails on the leaves. I had thrips once before and it looked similar. I sprayed with a mixture of potassium soap and neem oil to hopefully kill them (or whatever might be there).
Looking good, I need to get to the point where i can say "2 more weeks" confidently. Its more of a guessing game with the plants I've grown so far as there has never been enough really clear trichome changes, even harder to see on these purple varieties.
@eurorack, Thanks a lot! Just because I can say it confidently doesn't mean I'm right ;) There are other factors that also let me know that the plants are almost ready. They start to drink less water than before. Some of the fan leaves fade yellow/purple and some fall off. The pistils curl and darken. And the trichomes change as you mentioned.