Blkmane,
First, it depends on your multivitamin and whether, for your needs, there is enough B-Complex for you. With me, my multivitamin contains 25mg of all the B-Complex, including the co-factors. That's at the 3 tabs daily recommendation. I only take 2 tablets daily. With the B-Complex supplement which I take, I take 50mg am and 50mg pm. For my hair and the amount of stress I have in my life, that's cool for me.
Generally, with B-complex, you want to take it if you are taking any single B vitamins or co-factors. Otherwise, you can throw off your inner balance. With singles, you can generally take 2-3x the amount in the B-complex without throwing off your ratio internally. With that said, it is usually very had for most people to "get out of whack" here. Timing of dosages, length of time taken and your own body chemistry play major roles here.
From having taken nutrients for many years, I can also add this note. It is never the same taking a multivitamin for a specific issue which contains a particular nutrient which you want versus taking the nutrient by itself. The results are just not the same. In other words, if I wanted to take a B-complex for my hair, I personally would never rely on a multivitamin alone containing B-complex to take care of it.
Hope this helps.
~Jade~