Do you know what strain you have growing? You see, every plant has a different flowering time, so it's hard to say without knowing or seeing any pictures.

To know when your plant is done, you have to look at the trichomes through a small, 30x-100x, scope. These can be found at Radio Shack for about $10 for a 30x, $15 for an adjustable 60x-100x scope. You look at the color of the trichs, the crystals, at the heads of them. Through the scope, what you will see is something that looks like a mini mushroom. It has the stem, and a top. To know when to harvest though, you have to know what kind of a "high" or "stone" you are after.

For an up, energetic, speedy high, you want to harvest when the trichs are approx. 50% clear, 50% cloudy.
For a couchlock, narcotic type high, you would want to harvest when the trichs are approx. 50% cloudy, 50% amber.

I find the best time to harvest is when the trichs are 85% cloudy, 15% amber. When the trichs start to turn amber/red, the THC is begin to break down and they are very fragile at this point. Waiting any longer thasn 50% amber trichs, you'll just be losing potency. Hope this helped! Good luck!