A typical grow takes about 85 days from seed to harvest, but that number can vary quite a bit depending on what strain you're growing and how long you keep the plants growing vegetatively before flowering them. If you want to minimize calendar days of grow time, you can select early-maturing strains for growing.
If you're starting with a plant that's already sexually mature (eg a clone), then, again, depending on strain you can harvest in as little as 6 weeks after starting flowering.
On the other hand, some equatorial or tropical sativa strains may take fully 16 weeks, or even longer before the flowers are mature and optimum for harvest.
All of this assumes you're controlling the plants growth and flowering with artificial light.
If you're growing outdoors in sunlight, in the Northern hemisphere cannabis plants typically finish their flowering around October, irrespective of when they've been planted.
Again, certain strains can be done somewhat earlier, and some can take quite a bit longer.