How many hours should I use it?

You should use any number of hours that provides the most DLI your plant can handle. Local variables will dictate exactly what "max" is. whehther you supply this over 12 or 18 hours is mostly irrelevant. So, if you have a weak light and 100% isn't enough, more hours can help. Hours of operation is merely a viable tool to impact DLI given. I use a 16/8 schedule for vege because it keeps a larger portion of the electricity used with cheaper rates of electricty. Relative to 18h use, i have to amp my lights up about 11.1% to give the same DLI over 16hours. It's proportional... inversely to hours, of course. don't need a fancy 500 usd quantum meter. Just keep giving a bit more until the nodes are too tight, and that is the plant saying, "too much, dummy!" Take notes of power and hanging distance and it'll be very similar in the future. At this point a kLux measurement can be helpful too or one of those phone apps claiming to measure DLI/ppf/ppfd whatever misuse of vocabulary possible, lol. Temps, RH and atmospheric CO2 are the primary factors that will cause deviation. You may need tiny adjustments in future due to seasonal changes etc. Regardless, the growth pattern is your guide. DLI is a rate of photons produced x time per m^2. this makes it an apples to apples comparison to other gardens regardless of operating hours and area of coverage. read the wiki on it to get the gist. this is the proper way to quantify light applied to a plant.