I've always used 18. You can veg with anything from 18 to 24hrs of light but I wouldn't do less. IIRC I had a friend back in the day who vegged 10on 2off though. Not sure why or what the advantage was supposed to be. Anyone know?
Now, what I'm about to say has no real basis in science or horticulture that I know of...just my own own goofy thinking.
The way I see it...plants will sleep/rest for part of the day whether the lights are on or off. I personally have never noticed any significant increase in growth using 24 over 18hour days so I made up the part that they must be sleeping for about 6 hours with or without light. So, why spend money to waste power to run lights for an extra 6 hours per day that the plants probably can't use? we ALL have to sleep some time.?
IF on the other hand, I noticed a 30% increase in growth by going 24 instead of 18, you bet I would.
Also, just so you're thinking properly, and you already might know this...it isn't the shortened light cycle (from 18 or 24 to 12) that triggers flowering it's the UNINTERRUPTED 12 hours of complete darkness that triggers it. There IS a difference.
That helps to explain why 10/2 would work for veg. Also opens your mind up to even other possibilities like 18hour light cycle in flowering.
You know...if they can stay awake and growing for 18 hours, why not do 18/12 (with a digital timer of course based on a 30hr day) during flowering? They'll get the 12 hours of dark that they need to flower plus they can grow at their full potential for 18 hours like in veg.
Well there are a few reasons not to such as..what if you want to have a flip flop of 2 rooms flowering...one room on for 12hours and the other room comes on for 12 hours when the first goes out. Or the fact that you might get more growth per "day" on 18/12 but the days are now 30hours long and in the end, you'd probably end up taking the same amount of hours in the end.
It does help to think that it's the 12 of uninterrupted dark as opposed to the shortened day cycle though.
hope all that makes some sense and is somewhat helpful. Again...whether or not any of it is scientifically true..I don't know so take my info with a grain of salt.
YMMV