How long do b12 levels need to be low in order for back nerves/the spinal cord to be damaged? can that happen in as little as a few months to a year or would it take years before damage sets in? thank you?

Can happen in months. If you are deficient in vitamin B12, you could try sub-lingual formulation, though B12 injections are more effective. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV and Covid vaccines. Wish you good health!

Agree with Dr Singh. When B-12 levels drop too low, a slow evolution of symptoms commences and progresses. This key issue is to diagnose ASAP, and immediately commence supplementation. This does worsen if not addressed, but most of any initial damage can be turned around with timely intervention.