After they declare (Usually within weeks 3 - 5), it is roughly ANOTHER 7-9 weeks before they are mature enough to harvest.
The actual time varies, but a general timeframe is they tend to:
Declare gender around weeks 3 - 6,
Go into an intense bloom/growth stage for another 4 - 6 weeks.
Final 2 - 3 weeks - growth slows, calyxes on the buds really begin to swell...
Again, your milage will vary. I've gone from seed to smoke in under 90 days, then have had seeds from the same stock take upwards of 115 days total.
One thing I have found with autos, is it is better to harvest them a little late than a little early.