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?