It's legit, and you could make hundreds if you enough donate enough, but it's probably not gonna be that much. I did it a few years ago with CSL Plasma. They let you donate every two days (but no more than twice a week). Each donation took about 4-5 hours (2-3 waiting in line and 1-2 donating), and they paid me 50$-$150 because of sign up bonuses.

So if you managed to make 8 donations in the first month with the sign-up bonus, you could make over a thousand in a month. But after that, you'll make a lot less than that because it hurts to donate too much, and you'll likely get closer to the $50 side of payouts after the first few.

But ~$10/hr to sit with a needle in your arm while watching Netflix isn't terrible.