When ordering therapeutic phlebotomy you'll be asked to specify volume drawn (500mL is the default), weekly/biweekly/monthly draws, for a 12 month period. After 12 months a new prescription is required. According to an org specializing in blood iron disorders, the hemoglobin cutoff below which you should not draw is 12.5. The normal HGB range is 12-16 ish.
( notes tapped April 2013 for black book )

Russell B Marz, ND, MAcOM, lecturing
( notes week 6 after exam on nutrition )