I have read several posters mention a 6 month to 1 year recovery time in order to be completely recovered from low Vitamin D.
History: In January 2014, I was found to have Vitamin D level of 21.5 NG/ML. After I found this out, I researched my old blood tests and found that I had only been tested once before for Vitamin D. Five years ago, in July 2009, I had a Vit D level of 37 NG/ML ( which I know now was low normal). I believe I have been low for at least 2 years as my first symptom of rib tenderness or weakness occurred ~ 2 years before Jan 2014. By Dec 2013, I was having muscle weakness in arms, legs, back and rib areas. I also had severe fatigue, dizziness, and muscle twitching. Like many poster's comments, I experienced "felt like my body was dying", "had difficulty getting out of chair" and "had difficulty climbing stairs". I was put on 50,000 IU of D3 once a week for 24 weeks.
Now: After 16 weeks of D3 treatment I can say I am better, but is has been a very, very slow climb. My fatigue is practically gone and I rarely experience dizziness or muscle twitching. However, I still have some muscle weakness that waxes and wanes. Sometimes I feel almost normal and at other times my muscle weakness is noticeable. I still have some weakness in my legs and rib areas. I have read that muscle weakness can be the last item to recover from. I am back to riding my bicycle but I still don't feel 100%. Hopefully by the 6 month mark my muscle weakness will be completely resolved.