They are dry enough if stem cracks with a sound. Best way the slower the better. Beginning with high humidity in a room with humidity control. Around 2-3 weeks slowly decreasing it.
Taken from a Riley Reid's book "Growing Elite Marijuana"
Optimal Drying Environment: dark cool drying environment is critical. ANY light that touches your buds will at once begin to degrade it's THC content. In order to ensure optimum drying results, you are going to have to keep the temperature and humidity within a certain range for optimal results. Conditions should remain somewhere between the following ideal ranges;Temperature: between 65-76°F (18.3 to 24.4°C). Humidity: between 45%-55%. If the temperatures lower more than 65°F (18.3°C) the drying time will be lengthened. At temperatures higher than 75°F (23.8°C), the heat will cause the other portion of the bud to dry quicker than the inner part and as a result the taste will suffer.At a humidity lower than 45% the marijuana will dry way too fast and the taste will suffer. If the humidity rises higher than 55%, the marijuana will take a longtime to dry and will be prone to mold.