as far as light conditions go, I would not exceed 12 hours of darkness as this is pretty much the perfect threshold for all plants.
However, you may have a strain that does better with a longer dark period, and then and only then would I go beyond 12 hours of darkness.
If you do push it beyond 12 hours, you may not have any poor results, but it is likely that you could cause a female to become hermaphroditic because of the unnatural conditions.