Thanks for the question.

Essentially what you want to do is to simulate the day length your seedlings would naturally experience outdoors while also giving them enough intensity to grow sturdy and not leggy. For most vegetables and annual flowers (tomatoes, peppers, brassicas, zinnias, etc.), 12 to16 hours of LED light per day is ideal. This mimics long spring/summer days and keeps plants compact and healthy.

With respect to varying the amount of light exposure over a 3 to 4-month period, it’s best to keep the photoperiod consistent. Outdoors, the day length naturally increases as spring approaches, but indoors plants respond more to total light intensity than gradual seasonal shifts. Keeping a steady 14–16 hours/day ensures uniform growth and avoids stressing them.

A few other thoughts:

1). Put the lights on a simple outlet timer for 14–16 hours daily (e.g., 6 am–10 pm). This is far easier than you turning things on and off daily.

2). Always allow 8–10 hours of darkness. This will permit the necessary amount of plant respiration to occur.

3). When you move things outdoors, start by putting your seedlings in partial shade and then slowly increase the sun exposure over the next 7–10 days.

Here is some information on these and related points:

https://extension.umn.edu/planting-and-growing-guides/starting-seeds-indoors - :~:text=Standard fixtures with two "cool,turned off and on automatically.

https://hennepinmastergardeners.org/starting-seeds-indoors/ - :~:text=LIGHTING: For maximum growth, seeds,turn lights on and off.

https://extension.umn.edu/planting-and-growing-guides/lighting-indoor-plants

https://ag.purdue.edu/department/btny/ppdl/_media/publications/horticulture/ho-14-w.pdf

Good luck. Thanks for consulting us.