PI extracting is serious business. I went to the TEST server to see how much I can deplete the surface. My method was simply to chose a 24h program length and cancel it after two or more cycles. Then simply restarting it.
It is clear that the resources will get depleted and while I only mined for 2-4 cycles the whole amount of the program length was extracted. Below is a gif animation showing the resource map (skills are 4/5). The whole process took about 18h to deplete a WH Gas Planet.
I actually managed to deplete a part of the Planet to the absolute 0. :P
Edit: It's been a whole week since and the spot has not regenerated any resources. Might this be permadamage to the planet?