When you're harvesting the blood iron of your enemies you don't have time for ethics. A controlled study managed 4 weeks within ethical boundaries, maybe we can cut that in half unethically. So 14 days, 3000 . 14 = 115 years. So when you torture two people, you could manage to do 58 years. Better start early!

3 times a year? Where i live you cab give 1/2l every two weeks at a bloodbank.

And i can take more and by that activate a immune response to produce way more.

You're off on blood production. "Safe" would be once every six to eight weeks for a pint. Plasma can be taken a lot more often. The red blood cells take a while to replenish.

But let's get real. You're not concerned about your enemy feeling well. Force them to take iron supplements and take a pint every other week. They'll likely stay alive that way. Fuck em, right?

But remember they are your enemy so you could drain a bit more blood more often to shave a few years off maybe

No that would be impossible.