Marijuana is an annual, meaning it completes it's entire life cycle in less than one year.
Marijuana has 2 distinct phases: Vegatative--when the plant puts on mass, and flowering--when males grow balls (pollen sacs) and females frow buds.
Buds are ripe usually 8--10 weeks into flowering. After that the THC degrades and the plant dies.
Inside, using light cycle manipulation, it's possible to keep the same plant alive for years--in vegatative state (no buds).
If you want to pick buds year-round, you need a perpetual grow. Plant a few seeds (or better yet take root a few clones) every week or so.
But you need at least 2 grow area's to do this. One with the lights on 24/0 during veg, another with the lights 12/12 for flowering.