Yo devil boy
As mentioned above by the OG's, wait until most of the pistils turn brown and start curling into themselves, then you should be about 2 weeks from harvest.
Leaves are also a tell tale sign. Most leaves will turn yellow or at least have a proper fade to them.
Most effective still is to check trichomes. When 50%-70% of the trichomes has turned cloudy/milky and the rest have turned amber, you should harvest if you are looking for a stronger, more euphoric effect. When trichomes are ranging from amber to brown, this is when full maturity has been reached. Once trichomes turn amber they start to degrade in quality. By the time the plant is all amber and starting to turn brown, most of the THC has converted to CBN, which has a very relaxing, sleep aiding, couch lock effect. All is strain dependant and you have to experiment yourself to achieve your desired effect.