I decarb after extraction because I can tell exactly where I am at in the process by simply watching C02 bubble production.
Plant material decarbs naturally with time and it isn't possible to tell what percent of the plants THC-A has been converted to THC by look, touch, or smell, so a set time doesn't work, unless you do it worst case scenario, which is by definition too much for most of the material.
Sadly, the same temperature that converts THC-A to THC, also degrades existing THC to its decay products like CBN, which is less psychoactive and more sedative, producing more of a charley horse between the ears than a high.
If you look at the attached chart, you will note that after about 70% THC-A conversion, the rate of conversion to THC is less than the loss of THC to CBN, et al, so the total THC curves drop off.
In addition, the same temperature that decarboxylates the THC-A to THC, and the THC to CBN, also breaks down the CBN, so the total loss of THC will not be offset by a like increase in CBN, thus overall potency is less than when you started.
If you watch the bubbles, the production will dramatically drop off at about 70%, as the process curve peaks and starts down the other side. That is maximum THC level and time to stop if maximum THC is your goal. If you continue until there are no bubbles, you will have less THC , more CBN, and less cannabinoids in total.