Agriculture is about maintaining the optimal conditions of a crop for optimal growth. This means balancing a wide variety of parameters including humidity, light, airflow, irrigation, and nutrient dosing. Perfecting the balance is mutually exclusive to success: you cannot skimp or overcompensate on these parameters. For example, you could have the perfect conditions for lettuce, but have an under powered red-LED-heavy grow light and have terrible results.
The art and science of crop optimization has thus spent a lot of time and energy debating what kind of lighting conditions are best for each particular crop. So far the consensus is that the DLI (Daily Light Interval) is one of the most predictably effective method for estimate the light intensity AND duration for any given crop. The DLI is a function of the intensity of the photosynthetically active radiation (PAR) of a light source measured in PPFD and the amount of time the crop is subjected to this light schedule. A biological aspect is the role of light in a crop's reproductive/flowering cycle. For instance, a tomato plant will flower regardless of the length of the day, but a lettuce will only do so once the days get long enough and the temperatures rise. The particular variety of crop also indicates what light conditions are needed for germination. This is generally indicated on the seed package.
Through calculating DLI, you can make very informed changes to your lighting system/setup if needed. There are two basic scenarios here:
1. If you're gardening outdoors the lack of control doesn't leave you much choices. You must follow the patterns of nature, but you can at least filter the light intensity through shade clothes and reflective aluminet. In this case, stick to your gardener's almanac to decide when to plant crops.
2. Indoor gardening is where it gets more interesting in terms of lighting. The most important aspect here is to be very wary of the lighting you buy. Many cheap LEDs on amazon are lacking in the spectrum and may have unfavorable balances of blue and red LEDs. Cheap doesn't have to mean bad, however and there are a lot of companies with great LED products. A good amount of these companies have a strong online presence and have a lot of research and videos to back up their purported PAR output. Use these supplied intensity charts in tandem with the indicated hanging height to achieve best PAR estimate.
Ultimately you'll have to do a lot of experimenting as each individual light manufacture will have a slightly different spectrum but as a rule of thumb, blue light (4000+ K temp lights) favors vegetative growth and red light (<3000 K) tends to favor flowering. Depending on the type of plant, you results may vary!