How Long Does Ferritin Injection Take To Improve The Iron Levels In Body?

i have severe Restless Legs Syndrome. Last year I had an ALIF from L4-S1. My hgb pre op was 12.1; dropped to 7.4 post op. One month ago I had a second fusion from L3-4. Pre op my hgb was 9.4, while the ferritin level done at my request was 6. Tomorrow I have my first ferritin infusion. How long does this take? Poss side effects?