How long it takes to regenerated the blood after blood donation depends on the individual. And it also depends on what source you are citing.
According to American Red Cross, …
“The plasma from your donation is replaced within about 24 hours. Red cells need about four to six weeks for complete replacement. That’s why at least eight weeks are required between whole blood donations.”
According to Anemia.org …
“It takes a couple of days to replace plasma, the watery substance of your blood. Making red blood cells take a little longer, but a healthy donor can replace those that were donated within two weeks.”
Typical amount of blood taken when donating blood in the United States is 500 milli-Liters (about 1 pint and roughly 1 pound, or roughly 10% of your blood assuming typical amount of 5 liters of blood in body).
And it typically take 10 minutes for that amount to come out. But the prelim check and paper work can take more time. Plus after the donation, you need to wait at the facility for another 15 minutes to see if you are okay and don’t faint.
To avoid faintness, drink a lot of water or fruit juice before and after the donation — at least two glasses before the donation. But best to drink well within the 24 hours prior to donation. This helps keep the blood pressure up. Blood pressure drop is the primary cause of fainting.
When blood pressure drops, the body boosts it back up by having the adrenal glands secrete more aldosterone. Those with adrenal fatigue may have problems producing aldosterone and hence may be more prone to fainting during donation.
Also have a hearty meal before the donation (within three hours of donation). This keeps the blood sugar level up. And a hearty meal afterwards as well. Don’t do strenuous exercise for another 24 hours.