A great question is, how long have they been flowering? Now if you've had a period of stress this throws the whole timeline off. For many indicas, or hybrids with indica-dominant traits, the period of flowering is shorter, 60 days-ish; whereas, the flowering time of sativas, or hybrids with sativa-dominant traits, is typically a little longer. The difference in the amount of flowering time that is typical between an indica and a sativa is due to its location on the globe where the two different types of cannabis are "native" to- all cannabis is originally native to an area in the mountains of the middle-east near Pakistan. Shorter summers demand that indicas are given enough time to reproduce, yet they must finish before the freeze comes. Sativas typically have longer summers and milder autumns, so they can spend more time flowering. Knowing this should at least dial you into the right week of harvest, at which point you should start determining what kind of buzz you like...
As the last set of pistils turn colors, more white pistils left on the bud, say 1/3, then you are approaching max THCa production and the high, if dried/cured properly, should be more head-based or cerebral; and if you let most, say 2/3s, of the pistils turn colors before harvesting, then your high will be more of a body-buzz, or somatic, with a proper dry/cure. If you continue to leave your buds on the plant once most of the pistils have changed color, then your buds are losing their potency as THCa is broken down to less effective psychoactive cannabinoids:)