How long does to take to start getting the benefits of taking a multivitamin ?

Variable: This depends on what benefit you are talking about and your individual need. One potential benefit of a multivitamin is prevention of illnesses like heart disease & cancer- this might take a long time. But if you are deficient in some of the nutrients then benefits like more energy, better memory, reduced anxiety or improved immunity may begin within hours of starting to take it!

A long, long time: Unless you have a specific vitamin deficiency, or want specific support, like for pregnancy or menstruation, it will be a long time before you get any benefits from multivitamins. Take any specific vitamins you need, but multivitamins are a waste of money and potentially dangerous, and probably will never help your health. Save your money and buy a gym membership. Exercise has proven benefits.