Healthy individuals donating one unit of blood should be able to resume flying within 1 day since the fluid depleted is replaced within several hours. Any effects from the loss of hemoglobin should not be significant for normal flying operations.
Pilots are encouraged to consult a physician before volunteering as blood donors since it may take several weeks for the blood circulation to return to normal after a blood donation.
The FAA’s policy states “An airman who has donated 200cc. or greater blood for plasmapheresis or blood should not fly for at least 24 hours.” For new donation techniques such as “Double Red Cell Pherisis” it is suggested that pilots wait at least 48 hours before flying.