What is the Difference Between a ‘12 week strain’ vs. a ‘9 week strain?

The magic of Red Congolese & what makes it special:

Each plant has a flowering cycle, and for most of the production strains that you’re looking at these days, it’s an eight- to nine-week flowering cycle. But with the Red Congolese we go an extra three to four weeks in the growing schedule, which of course affects the number of harvests and your yield in a production year. It’s one of the things that makes us unique, but it’s also just what the plant needs to fully mature. Some straight sativas need fourteen or fifteen weeks, but twelve is pretty much the max — if it went any longer than that I don’t think it’s something we could grow. (source: Sparc)