There are three light schedules you can choose when growing in veg. For photoperiods, the majority give their plants an 18/6 light on/off schedule. Though 20/4 is gaining more traction and I’ve had great results from it. For autoflowering strains, growers seem evenly divided between 20/4 and a full 24 hours of light.
It’s recommended to place plants outside in the spring. You’ll want to make sure not to take them outside when there is 12 hours or more of darkness. And it completely fine to start growing them inside to give them a head start.
Both photoperiods and autoflowering will thrive under a full 24 hours, and Marijuana Horticulture Bible (pg. 38) states that this is the best light schedule. Though some growers like giving their plants some time off from the light and feel that their plants are less stressed.
Ed Rosenthal who is widely considered to be a growing guru recommends 24 hours of light and backs this will how cannabis plants photosynthesize.
“Marijuana plants photosynthesize as long as they receive light as well as water, air, nutrients and suitable temperature. Photosynthesis is the process in which plants use the energy from light (primarily in the blue and red spectrum's) to combine carbon dioxide (CO2) from the air and water (H2O) to make sugar while releasing oxygen to the air. Plants use sugars continuously to fuel metabolic processes (living) as well as for tissue building. The plant combines nitrogen (N) with the sugar to make amino acids, the building blocks of proteins. They are the substance of plant tissue. When the light is off, the plant's metabolic processes, respiration and growth, continue. The plant can photosynthesize continuously so it produces the most energy and growth when the light is on, continuously. Continuous light does not stress the plant, which reacts somewhat mechanistically to it. Plants under an 18-6 light-dark regimen are producing sugar only three quarters of the time. They are thus growing at only 75% of their potential. Leaving the light on continuously will result in bigger plants, faster, which leads to higher yields."
The question becomes do cannabis plants need darkness to rest and for other processes other than growing because most growers don’t see a full 25% increase of growth with 24 hr. It’s ultimately up to you and the strain you’re growing. I recommend to try different ones out and find the best. If you’re concerned about a high electric bill having an 18/6 will save you more money than a full 24 where you not only have the lights running, but a cooling system to remove heat.