Important Milestones in the Marijuana Plant's Life. Depending on how you set up your grow, it can take anywhere from 2 months to 6 months or more to grow a marijuana plant from a seedling to the point where the plant is ready to harvest.Some methods, such as growing hydroponically indoors, give your flexibility to get a harvested plant in as little as 2-3 months.ow long does it take to grow marijuana? From Day 1 of your marijuana plant's life to a Smokable Harvest, you're looking at 8.5 weeks - 7+ Months.