You know, there is such a thing as a useful approximation. Perfection is the enemy of the "good enough".
Between the Tropic Latitudes, day length does not change enough that almost no plants between these latitudes uses day length to know what time of year it is. Combine this thought with the fact that even the best & brightest of indoor light set ups come up to 25% of the intensity of natural full sun. Most indoor set ups are maybe 10% of natural full sun. The studies that recommend 18 hour day length do so recognizing that to some degree, you can compensate for lower indoor light intensities with longer day length. The upper limit is 18 hours, after which tests showed no increased biomass production with day lengths long than 18 hours. Plants from the tropics are usually totally insensitive to day length in terms of governing growth cycles.
Plants, including many succulents, from regions near the 30 to 35 parallels north and south are in the regions of monsoon influenced climates. Here the rains are sharply seasonal, and many areas have 3 month or longer dry seasons, and 3 month or longer monsoon seasons, the rainy seasons. Actual it is called a wet monsoon, and a dry monsoon. Between monsoons you can have more conventional, frontal boundary influenced weather. Again, day length is not normally the environmental cue the plants use to govern growth cycles. In these regions it is the presence or absence of rainfall that governs growth. OR it is night time temperatures that govern growth. During the dry monsoon, there will be no cloud cover at night, allowing cooling at night. During the wet monsoon, the clouds hold the heat at night. SO cooling off at night is normally the signal that plants in these regions will use to signal the arrival of the dry monsoon. (dry season) These areas do not have conventional winter, spring, summer, autumn. It is either the wet season, or the dry season or the transition season(s) between.
Sub-tropical and temperate species - some actually do use day length to know what time of year it is. Some use temperature, The actual number of documented species that are truly day length sensitive is small. Chrysanthemums are definitely day length sensitive, as the floral industry has documented. Poinsettia are also day length sensitive. Marijuana, Osmanthus, Papaver, possibly some Salvia are all day length sensitive. But the vast majority not listed are either more sensitive to temperature or other environmental cues.
Interesting factoid. Globally, from the equator to the poles, there are many trees, especially conifers, that volutize some 70% of the carbon captured by photosynthesis every day as volatile sesquiterpenes. Many of these sesquiterpenes do function as plant hormones, chemical signals, that can be read by neighboring plants. The haze that gives the name to the Blue Ridge Mountains is the sesquiterpenes gassed off by the red spruce, hemlock, balsam fir, and eastern white pines of the Blue Ridge Mountains. THat haze is a soup of chemical signals from one tree to another. There are large groups of plants that have abandoned investing the chemical energy required to run biological clocks. Instead, these plants simply "read" the chemical signals the local conifers are sending, and they respond the way their neighbors do to the environment. This is why plants taken from south of the equator to north of the equator very quickly readjust their clocks to align their growth to the seasons they are currently residing in. They pick up the signals from the trees around them. So the South African spring flowering bulb will begin flowering in spring in North America in as little as 18 months from crossing the equator. It is the chemical signals floating in the air that tell the bulb what time of year it is.
SO
@penumbra is correct - SOME trees are indeed daylength sensitive. And it is useful, especially if you are deliberately trying to keep your trees dormant, you should use less than 10 hours of day length. If I were to put a light in my well house, where it stays cool and I want dormancy, I would set the light for only 6 or 8 hours of light a day.
But the over all number of trees that are documented to be day length sensitive is really low. There is a somewhat larger number that are suspected as being day length sensitive, but not documented. For example, it is assumed most of the Aster (including Chrysanthemum) family are day length sensitive. The overall number of species is unclear, and thought to be relatively low.
If one is keeping temperatures up, and is looking for growth over winter, then the 18 hour day length is definitely the way to go.