Easy to tell, you should have drainage holes punched into the bottom of the cups. When you see roots in those holes it's time to transplant
You rock, WCC; thanks again for another great piece of advice.I run a pre veg cycle that takes place in solo cups or 4" pots. They are well rooted when I transfer them from our aerocloner into the cups. I feed 1/4 strength veg nutes the first week and 1/2 strength the 2nd week. They are under 8 bulbs T5's and remain this way until I transfer them at week 3 into their final container, usually a 5-7 gallon root friendly container for indoor. So yeah my clones get a pre veg in cups for two weeks, any longer and it's just spinning your wheels...
W.C.C.