Don't put them under 24 hour light, they will almost certainly flower once they go outside if you do that. You should not even root your clones under 24 hours light or keep your mother plants under 24 hours of light if you want to put them outside, it will often cause issues. You need to look at how much light your spot will get at your planting date (between May 21 and June 21 works for most locations and strains) and then slowly reduce the amount of light by no more than 15 minutes per week until the light cycles match up. If you drop them by 1 hour per night they will flower on you, that is far to much far to fast. Reducing all the way down to 14 hours will make them flower almost for sure as well. Most strains cannot go below 15 hours, give or take, without flowering.
Right now my mother plants are on 18 hours of light. Starting this week I will be dropping them by 15 minutes per week (subsequent clones will be on the same light cycle). By the last week of May I'll be down to 15 hours of light total - the same amount they will be getting once I move them outside. Then they go for 1 week to harden off outside for a June 1 planting. Works 95% of the time for just about every strain.
Also the strain does play a huge part in this. Earlier strains flower easier and are more difficult to adjust. Later strains are much easier to adjust. Generally any strain that flowers before Oct. 1 is going to be difficult to adjust for outdoor.
Also Spider K is correct about getting a really early start not having any affect on yield. If anything, it will hurt your yield. You want, young, healthy plants, not old, big plants when you move them outside. I would not start clones any earlier than Mar 15, but don't go later than April 15. The size of the plant when it goes out has little to no affect on final size and yield.
