How long does it take your body to absorb b complex vitamins?

Only minutes. In most cases b complex will be absorbed quite quickly. One way of verifying this is to see how long it takes between taking a potent b complex and how long it takes to see your urine turn dark yellow & smell like vitamins (caused by b1 & b2). For them to appear in your urine they must be absorbed into your bloodstream and then be filtered by your kidneys, but this can happen within an hour.