Gave a lot as a teen. Quit when I got a letter that I was no longer eligible as I had some hideous blood disease. Turned out I didn't, gave up. Now I'm just to old to weaken myself like that. Already shitting blood from hemorrhoids, just now getting that under control.
Funny story, went to the doc because walking upstairs to my office was making my heart pound. She sent me to the ER, they took blood, "OK Mr. Shalafi! We have a room ready for you on the 4th fl..." "HOLD UP! I'm staying?!" I was within an inch of needing a transfusion.
Knew I was squirting a little blood sometimes when I pooped. Explained I didn't think it was a big deal as a human can give a pint a week at the Red Cross. "Uh, no. You can give a pint once a month." Oh.
I got a lot of extra blood but it seems like a big deal to go and donate. Does it hurt? Do you get a lollipop afterwards?
You should read about it on the website of your local donation center/organisation. They probably have the best info.
Where I'm from you get a cookie!