Hi, thanks for the comments. I caught the first comment that you left, but didn't have time to respond.
My plants (there are two in the box) are 18 inches tall at the highest point. I still have about a foot of grow height left in the box, and I'm still LSTing as they grow. I have always been really concerned with the height issues I may face, but so far I haven't run into many complications. I simply tie the longer branches down when they punch through the canopy. Also, the main stem of both plants is probably close to 2 ft, but both have been LST'd to well below 12 in.
To answer your original question - I'm not sure what I'll do if they outgrow the box.. Maybe harvest them early, I'm not sure. I hope it doesn't come to that.
I also take your point about light leaks. I did the best I could to seal it, but it's possible that a minimal amount creeps in every so often. I absolutely agree that as outdoor plants are subjected to moonlight which doesn't do them much harm, the same principle should apply to indoor grows. As this is my first grow, I could be completely wrong about this - I'll soon find out.
So far so good, though. Loads of pistils shooting out of the buds. No pollen sacks. Knock on wood, things continue this way.
EDIT:
It might not look like I have a spare foot of height from the pics I posted, but that is simply because the lights are hung about 10 in from the box ceiling . I can raise them as the plants grow. Fingers crossed a foot will be sufficient.