I'm personally of the opinion that 90% of the growers on this site (And probably in general) harvest too early. By letting the plants go a little longer you're ensuring that the've plumped as much as they're going to.
Despite popular opinion to the contrary(and don't fool yourself, the jury is still out on this one) Recent studies have shown ;contrary to what was previously believed, that THC itself (And its predecessor THC-A) are quite guilty of causing the confusion and drowsyness associated with burnout and other cannabinoids (our friend CBN, and a handful of others) to be the catylist (along with THC) to being 'high' rather than 'baked'
Regardless, whether or not theres any substance to the aforementioned study, its easy to get the high you want.
If you want a soaring 'cerebral' high: Get yourself a tropical sativa that contains a high level of THC-V and grow it until it is ripe.
If you want the narcotic couchlock stone, grow a rugged indica until its ripe.
Notice a pattern of growing it until its ripe? Its a really good rule to live by.
OF course we have to remember that there is more to a good high than just THC. At last count there are at least 66 cannabinoids, and we don't know what most of them do.
Now alot of people will tell you that you should harvest based on the colour of your trichomes. But (again, in my opinion) that is far too simplistic and there are too many variables to make that an effective strategy. I've made that point a million times before and i'm not going to re-hash it here, but rest assured there is more to the picture than just trich colour.
A ripe
marijuana plant will be filled in, will have an amber tinge to the buds. The pistils should have browned (or orange-d) off and receded into the buds. The seed bracts should be swollen and the trichomes should be sticking straight out with bulbous ends. Also, because you're coming close to the end of plant's life cycle, the leaves should have yellowed off and started to die.
one of my plants @ 10 weeks, 3 days before chop-chop.
Another very important (imho) reason to let your plants mature is Terpenoid production. Terpenes are responsible for alot of the complex (And enjoyable) flavours that cannabis produces. Some of the most intense flavours are produced on the "downslope" of cannabis's life cycle. My personal experience with this came when i was growing GH cheese. One of the plants i harvested at 8 weeks and it tasted pretty nice and had pretty dense buds. The other i grew until about 9 1/2 weeks and it had most amazing, sour, skunky, delicious taste with
rock hard buds.