Usually when the fan leaves reach the edges of the cup, when you have to water every day, or you start seeing roots come out the bottom its time to transplant. As far as soil i would go with foxfarm happy frog for your 1st transplant. You can go directly into your final pot 5-7 Gallon but if You choose this method be careful not to over water , you just want to drench the center of the pot( Use this method for auto-flowers) The 2nd method is to go from solo cup to a 1 gallon pot and down the road switch to the slightly stronger fox farm ocean forrest soil in the 5-7 gallon. I prefer this method because everytime you transplant you have new soil that will feed your plants and keeps you from having to feed for about 30 days. Also when transplanting sprinkle mycorrhizae on root ball and in the hole