You're right about the lead time - I turned vegan in May 2017 and began to have symptoms of osteoporosis around 14 months later - great joint pain. Turned out my vitamin D levels had plummeted to critical as I didn't take care to replace the Vit D that comes in an omnivorous diet, and had some periods of not going out enough for work reasons. I currently take 10,000 iu/s a day of Vit D3 under medical supervision, and the recovery has taken longer than the decline.