4-5 years. Pure vegans will develop vitamin B12 deficiency in 4-5 years if they do not take vitamin B12 supplements. It usually will take 4 years for the bone marrow to be completely depleted of viitamin b12, which ultimately will lead to anemia. Vitamin B12 is predominately found in meat products.
Vegan and nutrition. While it is true that if you only eat plant food you can be at risk to develop vit B12 deficiency& if it's not treated you can develop anemia also other neurological problems which could be irreversible- however, this is something you can prevent. If you decide to become a pure vegan - you should take vit B12 supplement. You wont become malnourished if you become vegetarian, if you eat right.
Brain damage. "Vegetarian's myelopathy", damage to the nervous system from lack of B12 first presenting as tingling or numbness of the fingers and toes, has been reported as early as two months following onset of a nonsupplemented vegan diet. The leaders of the vegan community urge their people to supplement wisely and if they do, the lifestyle is safe and of course gives the moral high ground.