Average 3 months. "most relevant studies show that approximately 20% of the rbcs are lost in the first 24 hr after transfusion. The average life span is 94 days after a storage period of 42-49 days." this can be much shorter depending on the underlying medical condition that may cause ongoing hemolysis. Ref: cell mol biol (noisy-le-grand). 2004 mar;50 (2):197-203. Survival of the fittest?
Less than a month.. It depends on the age of the transfused blood. The normal life span of a red blood cell is about 30 days before it is destroyed in the spleen. Transfused blood is usually not as hearty as natural blood so I would expect it to degrade less than 30 days.