grapefruit outdoor grow help...deep south 2012

I got a few grapefruit/diesel girls going in some red solo cups. They r about 2 inches tall now and r just now starting on th 2nd set. How long do yall think I should let them stay in the cups? This is a outoor grow btw. Any suggestipns? This, isa fd2blk seeds

Transplant when the roots reach the bottom of the cup.

I appreciate your help. Im pretty new to using cups.

Cindy. That dog is AWESOME lololol