WHat various signals are there of a need to transplant? for example the roots to stem ratio???
Here's a more basic way of looking at the matter:
Bigger pot = bigger root system
Bigger root system = bigger plant
Seedlings should generally be transplanted in about 2 weeks from initial planting after germination. Next, decide how tall you want your plant to be. Also figure about a gallon per foot with regard to size of the pot you will need(or perhaps already have). Ballpark figures here, so your results may vary.
Not sure if any of this helps, but it's just a different view of the problem. Maybe there are growers that can tell you exactly when to transplant based solely on the physical charicteristics of the plant, but I sure as hell ain't one of them. It's kind of like boiling water. I know it's going to take about 5 minutes, so there's no need for me to stand there with a thermometer and wait for it to reach 212 degrees.