There was a thread at another site where some peeps did a test on this. They found that 20/4 had the best results, 18/6 was also good, but 24/0 was the worst (if I remember that last one correctly). They tried other combinations too, I'm sure you can find the thread if you look for it.

It is probably somewhat strain dependent also, but yeah, 18/6 is usually the norm, can save a lot of money when running a lot of watts.

EDIT: Found it!

Originally Posted by ot1
We did a lot of experiments with light times a few years back using known sat and sat dom clone lines.

With Vegging ie under hid lights.

20/4 produced the most sturdy growth and the most bulk. Best final yield, taken as 100%.
22/2 Less of both. yield 88%
18/6 Studier than 22/2 but slightly less bulk. yield 87%
24/0 Much lighter in all aspects than 18/6. Yield 79%
16/8 The weediest plants. yield 67%

Plants vegged to final pots under fluorescents at 20w per sq ft on 18/6 yield 49%

For the mum lines we have 20/4 to 12/12 gives the best crop weight and bud quality, really thats all Iā??m interested in.