Before you transplant Grandpa, I want you to hold your cup with plain jane in it and I want you to just gently guide your finger along the rim of your cup. Slowly, you will feel that the leaves of plain jane overlap the edge of your cup. Now record that in your sensory.
Your cup has a curved edge, so put your finger along the soil line too. It covers a smaller area space than the rim of your cup. Don't be afraid to get your fingers dirty. You can actually do this transplant totally on your own, you got this!
This is how you can tell if your seedlings need transplanting. Those young roots are searching for water and wanting to grow. When I look at your cup I see that it's time to up pot/transplant, but how would you know for the future? Use this technique, because you don't want your plant to reach beyond her edge too much. If you feel yours, you can feel about 20% (not accurate) of the leaves are hanging over.
Now this technique doesn't apply in late veg and flower because you want them to overgrow the edge. What you are looking for is a consecutive outgrowing of your pot till it's time to flower. This shows active and fast growth for you to touch and learn. When we get you through the cycles of this plant, you can start touching your plant and start to figure out how old she really is by touching the length of your stalks, and wideness of your flowers.
I know it's obvious to us, but now that you're growing fem's, you will have the chance to learn about growth stages easier than with an auto.
Hope this helps...