How long does it take for vitamin d injection to work?

I don't know what you mean by "work". Blood levels will go up immediately and then fall as it is excreted in urine and stool and is stored in your fat. Discuss with your Dr who gave you the injection. Rarely done in US. as innections are very painful. So need to get blood levels rechecked after a month or so for Vit D. I recommend going to uptodate.com and spend $20 so you learn all about Vit D.

If your vitamin D is too low, would typically advise oral supplementation of 2,000-5,000 IU's daily and have never given any patients a Vitamin D injection. Since it is fat soluble, it would take about 3 months to stabilize, and at that time, you could check serum levels. Aim for eventual 50-70 ng/ml.