We've done a couple hundred tests, and one of the reason I came to dispel some myths, like I've seen here and everywhere. For starters, it is not true that you should only use UVB in the last couple of weeks. You CAN do it that way, and get a nice bump in trichomes, but you have to dig a little deeper and understand more about why we are using UVB to begin with: UVR8.
I recommend starting UVB in the 280-290nm region as soon as you flip. Don't wait. The reason is that we are trying to trigger the UVR8 protein from day one of flower. UVR8 is a photosensor, but it doesn't measure day/night length, it measures UVB, and is most sensitive from 280-290nm. This is why tanning bulbs and reptile lights aren't very effective, they stay above 300nm. Btw, all plants have the UVR8 protein, not just cannabis. They all react differently to the chemical signals that the protein sends to the plant. Tomatoes develop thicker flesh and more flavonoids. Sorghum matures faster, so you can get 4 cuttings a year instead of 3. Cannabis creates more trichomes/THC, but ONLY because THCa has a super high absorption index for UV. Basically, UVR8 says "Hey, we are getting pounded by UV" and the mother plant reacts by coating the buds because she thinks she's going to have seeds in there, and exposing seeds to UV will cause DNA damage and reduce how many are viable. This is why breeders should NOT use UVB, but everyone else should.
So you start from day one of flower to keep the plant in defense mode during the entire flowering season. The other reasons you do this are to kill PM before it starts, when it is still a spore, and reduce stretch, reducing internodal distances on the plant. On average, a plant that is hammered with our Flower Power UVB lamps for 2-3 hours a day will be around 6" shorter than ones that weren't, based on testing with clones. This varies by strain, but regardless, it is noticeable. This is because the plant is in "defense" mode, and is reallocating resources from growing stems, to growing trichomes. There is no difference in yield, up or down, it's just higher in THC. Going from 18 to 26%, 22 to 29% and 26 to 34% is certainly doable and is being done by experienced growers with a little experience with the bulbs, pushing the times up a bit. You don't get that by only using UVB in the last two weeks. How you do that, I will leave for another post, this one is long enough as it is ;)