started from seeds need help

growing in dixie cups 16/18 oz. when can you tell that the plant needs to
be transplanted?

If they are clear you can tell where the roots are...but then again roots should be in complete darkness. I would wait untill it gets at least a week or two of growth first. I tranplanted mine after a week and it lived up untill its second transplant. But let it get as big as possible/ when growth slows transfer to a bigger pot.