"how long does it take to become B12 deficient? Let's say all you eat is fruit and veg, how long would it take? And, would it necessarily occur at all

B12 foods. Eggs, cheese, beef and seafood have a lot of b12. Plant foods contain very little (if any) b12. Do not contain vitamin B12 except when added to them. Thus, vegans need to look to fortified foods or supplements to get vitamin B12 in their diet.