From what I've read the is no definite period of time until harvest. Every strain is different. Between 6-12 weeks after flowering you should be ready for harvest. While there are estimates its best to look for visual clues. When the white hairs start turning red, orange, or brown you know its about time to harvest. Once most of the hairs turn color its time. (but some people harvest early to obtain a different type of high).
Now I have been very interested in why people say "this weed gives you a couchlock high" or "this weed gives you an energetic high." I'm a first time grower and I'm very interested in my weed giving me an entergetic high. After research I've found that the types of highs you get from your weed is directly related to harvest time. The levels of THC and CBD are affected by when you harvest.
Weed with high levels of THC and low levels of CBD will give you a more clear energetic high. Higher levels of CBD will give you more of a body high (I guess couchlock). I've read that weed with high levels of CBD can even give you a high that makes you stupid (I'd rather be clear headed).
If you harvest early the THC will be higher. If you wait longer to harvest the THC will start breaking down into CBD. So you should decide on a harvest time taking into account what type of high you want your weed to give.
In summary I'd say harvest when the hairs are clear or milky white (or right when you see the first white hair turning color), to obtain a more cerebral high or energetic high. Or you can wait until the hairs start turning red or orange (darker) to get more of the couchlock effect or body high.