height of plant doesnt directly determine transplant time, but indicates you should be looking for other signs that you need to. if the growth rate slows down and the plant isnt thriving i'd give it a transplant. and the new container it is going into shouldnt be so large that the SOG still covers roughly 70% of the soil area on top. thats a rough number to play with over time. but if too much soil sees the light, the water will just evaporate out. any pot smaller and it becomes almost a redundant transplant.


but after a closer look at your picutres, i would say they are quite large for party cups, go for the transplant. Be careful! and Good luck sir!!!!