First Grow - Light Cycle Advice

Well this will be my first grow and already off to a late start. What I'm worried about is light cycle. I plan to start my grow indoors and move to my plot when they look strong enough to fend from the wild, but because I'm running a bit behind my plans I most likely won't be able to transplant them to there new home until the decrease in daylight. My question is what would you suggest be better to start their cycle indoors to help combat the change to my plot. 12hrs - 18hrs - 24hrs. Any advice would be so very appreciated.
Thanks so much.

Thanks Twistyman, I think I might start with 24hrs too. Would anyone recommend a minimum size before transplanting to my plot. I'd like to veg them inside as long as I possibly could at 24hrs but will have a pretty hardtime transporting foot tall plants their new home.