Some science :
Results of Decarboxylation Experiment
The following charts show the results of the 30 minute and 60 minute decarboxylation experiments. Also included are the lab results from testing done prior to any artificial decarboxylation to establish a starting point. Note that because of the age of both the kief and the trim, decarboxylation had begun to take place to some degree naturally. This may not be your starting point, but should not affect the results of the experiment much.
Kief
Compound Before Decarb 30 Min Decarb 60 Min Decarb
THCA 24.5% 2.6% .1%
THC 3.8% 25.4% 25.5%
CBDA .6% .3% .3%
CBD 0% 1% .1%
CBN .4% 1% 1.4%
Moisture 0% 0% 0%
Total Cannabanoids 29.3% 30.3% 27.4%
Cannabis Trim
Compound Before Decarb 30 Min Decarb 60 Min Decarb
THCA 6.5% 2.9% .2%
THC .6% 4.8% 6.9%
CBDA .2% .2% .1%
CBD 0% 0% .1%
CBN 0% 0% 0%
Moisture 3.4% 4.5% 0%
Total Cannabanoids 7.3% 7.9% 7.3%
Testing provided by SC Labs
As you can see from the two charts, 30 minutes was not quite enough to completely decarboxylate either the kief or the trim. At 30 minutes the kief was about 90% decarboxylated but the trim was only about 60% decarboxylated. This difference is likely because the trim had a higher starting moisture content. After 60 minutes however, both keif and trim samples were close enough to 100% decarboxylation for my satisfaction.
So there you have it. 240° F for 60 minutes should be enough to decarboxylate any cannabis with a reasonably low moisture content. For material with higher moisture content, the time can be extended but the temperature should not be increased. If you are concerned about losing organic compounds, lower heat can be used but the time should be extended to compensate.
Extracted from http://www.marijuanagrowershq.com/decarboxylating-cannabis-turning-thca-into-thc/