I've never had an outdoor harvest, so I cant really comment on how long >.<
I would keep an eye on your flowers though, and your fan leaves. You will notice shifts in the color. Your flowers hairs will start to change to a red, and late in life your fan leaves might start turning yellow. This is natures way of telling you your girls getting ready!
Do you have a loupe? Or some kind of scope to get a close up look at your flowers trichs? They will be clear early on in your plants life, and will start to turn milky. Usually people harvest when they have mostly milky trichs, if they turn amber/brown they are starting to degrade a bit.