The short answer to the very general question of how long it takes to grow weed is around 3 months. This is just an average number and is accounting for around 1 month in the vegetative stage followed by 2 months in 12/12 light schedule for flowering. This is not taking into consideration preferences or the other countless factors that you have with different strains and different grow room set ups. I’ve never gone more than a month and a half in the vegetative stage but know that patience is very beneficial when it comes to yields. The plant will get most of it’s leafy and structural growth in this stage and the longer it is drawn out the larger it gets to an extent. However, you could also switch over to a flowering life cycle very early on to initiate a faster harvest. This is also ideal if you are limited on space or are growing from a clone instead of starting from the seed.
That is where the 3 month answer stems from. It’s just a general rule of thumb. It is very important to either know the exact genetics and strain information or have access to equipment that allows you to examine the trichomes extremely close. If you know the strain, you know how long it will have to be in flowering to get the appropriate bud development. Most strains are around 8 weeks while hazes and sativas tend to take longer and up to 11 or 12 weeks sometimes. This is of course if you are strictly controlling the environments temperature, humidity, and other growing conditions.
This time frame is not taking into account drying and curing which adds a substantial amount of time before your medicine is ready to be consumed. Drying only takes around 4 or so days where curing takes at least 2 weeks. Many growers cure longer then 2 weeks due to the improved taste that comes from properly cured buds. To be a grower you have to be patient, but if you do it right the wait will make the crop that much more rewarding then purchasing it every time you want to consume.
Hope this quick answer gave you an idea of how long you should expect to spend during the satisfying grow proccess!