Welcome, Rock!
A couple of things to help you out. There are two kinds of cannabis that you can grow. Photo plants and auto plants.
Photo plants - these plants will start the flower when the light/dark timing is 12/12. You can have the lights on 24 hours a day and they will not flower - just continue to grow. This is called the vegetative stage. Once you 'flip' the lights, the flowering has begun. Each strain is different but the quickest is probably 7-8 weeks in flower up to 12 or more.
Auto plants - these guys were mixed with ruderalis, a cannabis relative that will automatically begin to flower in about 4-5 weeks. As with photos, they will typically start at 7 weeks to flower but more likely 8 - 12.
When you read about strains and they say flowering is 8-10 weeks, that is from when you flip the lights on a photo strain. If it's an auto - it would be from seed to harvest.
Hope that helps