Where I am at in the middle of the country, you can donate blood every 60 to 90 days (I am not sure which, but definitely no more than 90 days).
The blood bank here is VERY aggressive at calling and texting and emailing you, reminding you that you are eligible to donate again, basically RIGHT ON THE DAY that you can donate again. Like
@Roger rabbit said, it's a business for them, and a very lucrative one. Which is why I get annoyed when they use the emotional ploy of guilting you into donating, as if they are just doing this to help the local community.