Don't even think about TP'ing until the plant's widest leaves grow beyond the pot rim. I usually wait until the plant is 2 - 2.5 as tall as the pot. That should be an effective rule of thumb.
Don't worry about visible roots unless they start to turn green or black. A root ball is a sign of a healthy plant.
I like to wait to transplant until I feel the roots have completely filled the container. Here's one I just transplanted, it's been 4 weeks since it sprouted.