I see that multivitamin pack contains tablets for 60 days. Should I buy a new one once this finishes? I particularly ask because one product contains 1200% RDA of B12. How long can you sustain that?
The potential toxicity of vitamins varies enormously, but as a rough guide, fat soluble ones have a much higher potential to be stored in your body and build up to toxic levels whereas water soluble ones are likely to simply be flushed out in your urine. I would be wary of taking high doses of fat soluble vitamins over an extended period. See here.