How long until my black toenail falls off and a new one starts growing?

More info needed. To receive an answer that will be more helpful for you , you need to give more information...Your health status, any problems such as gout, diabetes or blood pressure concerns.You should state how long it has been a problem and what you do to make it feel better.Also tells us what it looks like..Is it discolored, swollen or point tender.The more you tell the better the answer.
4 to 6 months. Faster in younger patients and in warmer weather...Slower in older patients with less blood flow, in smokers and in colder weather. The nail may not fall off. You may just see a new clear line as the dark nail grows forward.
Approximately 1 year. Depending on how it was injured it may take up to 1 year for the nail to fully grow out.