The question of when do I transplant out of solo cups comes up all the time.
This pic sums it up: When the roots poke out the bottom drain holes!
I transplant from solo cup into 2 gallon fabric pots for vegetation, and then into a 5 gallon fabric pot a week or so before flowering.
YES – you will eventually start to see healthy white roots growing out through the bottom of solo cups. They will also grow out the bottom and sides of pete-pucks, rock-wool, and fabric pots!!!
NO – Cannabis does not like being transplanted. BE GENTLE with the root ball when transplanting!
For those who want to know how I make the holes: Solo-Cups for Seed starting: Drain-hole Tutorial