I've been growing sprouts for the last 5 years,I usually have 10 to 12 plants. what ohiotiller and gardenvt say is true, they take a long time to grow to full maturity.The first few frosts really sweeten them up. I usually begin eating mine at Thanksgiving and Christmas. I think you can leave them on longer but I never have, well they don't make it any longer as they get all eaten up.. .
Last years crop grew to just over 4' tall each. With lots of sprouts, Some marble size some 50cent size,
I give them away at Thanksgiving, ( I let family and friends pick a plant)
Just keep fertlizing as the season goes on (this gets hard as its usually the only thing left in the garden) and don't get discouraged.
Mine now are just about 2' tall and are showing sprouts, I planted mine in early April, The way the winter was I probably could have planted them in March.
Good luck and enjoy in the fall..