Id say beginning. alot of growers will start their seeds in solo cups, once the leaves are nearly at the edges its time to transplant. from there you can move it into larger container (and do that again) or move it to its final resting place. its up to you. my first grow i just planted directly in its resting place. this time around ive used a small pot (bigger than a solo) and now moved it into its final resting place. they are a little older than 3 weeks old from seed.
i'm not sure if when you should transplant based on when your light turns on/ off - if thats what you're asking, i may have misinterpreted.
good luck!