According to American Red Cross Society
You must wait at least eight weeks (56 days) between donations of whole blood and 16 weeks (112 days) between Power Red donations. Platelet apheresis donors may give every 7 days up to 24 times per year. Regulations are different for those giving blood for themselves (autologous donors).
It can vary a bit depending on your general health, diet which may effect your hemoglobin level, where a minimum of 12.5 g/dL is required every time you donate.