It’s definitely ready for a bigger pot, I only leave sprouts in a cup 10-14 days, that’s about how it takes for a healthy sprout’s roots to start growing through the holes in the bottom. I’d move it into a 1 gallon pot for 4wks then to a 3-5 gallon pot for a few wks before moving into the 15 gallon container. You could go straight to the 15 gallon but you’ll need to be very careful not to overwater that way and the growth will be pretty slow at first. 15 gallon pots are huge, unless you’re moving it outside you won’t need that big of a pot. Indoors I go from a cup to a 1 gallon then finish in a 7 gallon pot.
