How long before I can see hair growth from using 15000 mcg of biotin...?

Maybe never. Biotin deficiency is rare, but can occur in persons with genetic mutations, on long-term antibiotics or anti-convulsants, or who eat a lot of raw egg whites. There is no good evidence that this easily available, over-the-counter supplement is the magic potion to grow hair or prevent hair loss. Biotin is safe to take, but 15000 mcg per day is over 100 times the usual dose, so is not advisable.