Artificial lighting

[raybark]

There certainly is no set answer that applies universally.

This may seem more knowledgeable than I truly am on the subject, so take this only as a series of potentially-pertinent comments to act as a "jumping off point" for further thought and inquiries.

•Not all plants react to photoperiod for the initiation of blooming. Many phalaenopsis, for example, are triggered by changes in average temperature.
•In some cattleyas that do respond to photoperiod, it is a reduction in day length that triggers blooming, not extension of it.
•A lack of sufficient light is often a significant contributor to plants not blooming, so if your latitude results in short days, supplemental lighting might be the key.
•Plants need a certain amount of "down time" from light exposure, so that biological processes other than photosynthesis can take precedence. I'd bet that David's supposition about geographic origin as it relates to day length is appropriate.
•You also need to consider the lights themselves - wattage, age, and distance from the plants. Most fluorescent tubes should be replaced annually, and often need to be closer than we may think. I have a 200W CFL in my greenhouse, situated in the center, about 6 feet from the closest plants, and it adds very little to the light flux even those plants see; probably nothing to most of the GH (it's fine for working out there though). I'll measure what it adds and repost later.

With all that to think about, what I have usually read is 12-14 hours on.