All plants are different, and it depends on how picky you would like to be. But when to harvest is a big question, as it will really affect the quality of your bud and the type of high you end up with. Its common knowledge that indicas have a more body stone & sativas have a more heady high, but this can be altered by harvest times to an extent.
But heres the basics first. Most indicas are more picky about when you harvest than sativas. Most take about 8 weeks to flower, and the harvesting window is about 2 weeks, unless you want to entirely ruin your bud. Sativas on the other hand take anywhere from 8 weeks (with the new varieties) to 15 weeks (especially African strains) to flower, and theyre harder to mess up (have a wider window) due to their slower maturation time.
So, easiest way to guage harvest time is by the hairs on the buds. Most people say harvest once 60-75% (but everyones different) of the hairs have turned red/orange.
By getting yourself into this, you can better control the high and guage the maturity of your plant. You can tell the levels of THC, CBN and CBD in your bud this way.
Your trichs will start off clear, go milky, then turn amber. The earlier you pick, the more milky (not clear) your trichs will be, and the less CBN and CBD there will be in your bud. You will still have the high THC levels, and if you like an up, cerebral sativa high, you will want to pick your buds earlier while some are milky and some are amber.
Then if you wait longer until the trichs are more amber, the THC levels will begin to drop and the CBN and CBD levels will risebringing out a more couchlock stone.
So anyways, sorry to ramble. Most people go with the hairs, but I like my little loupe. It helps me feel as though Im really important or something. Lol.