Will piperine endure heating for several days of soxhlet extractions?
So, I promised to answer this question based on my experiences if I'd remember. And I did. Here we go:

I did three soxhlet extractions, each with 175 grams of black peppercorns. 

The first extraction took 6 hours and the cycles were quite slow due to the vapor pathways of the apparatus not being insulated.
The second extraction took 2 hours, again with an uninsulated apparatus.
The third extraction also took 2 hours, but the cycles were much faster because I used aluminum foil for insulating the apparatus.

Each extraction used 400 ml of 99% isopropyl alcohol of which 200 ml was distilled off after extraction. Something to note is that this reclamation for the first run took at least two times as long as it did for the two later ones. 

The yields were as follows:
- Run 1: 8,6 grams of piperine
- Run 2: 6,8 grams of piperine
- Run 3: 7,5 grams of piperine

From this I concluded that with sufficient insulation, something like 3-4 hours could potentially provide the optimal efficiency and that the yields of piperine aren't significantly affected by prolonged periods of heating in an alcoholic solution.

I hope this write-up will help or be of interest to someone!