B vitamins are water-soluble & generally absorbed pretty rapidly. If you are taking a "mega dose" of Bs beyond the RDA, you may notice that within a few hours, your urine becomes bright yellow. That tells you that the vitamins have gone through your system.

B vites do not work their miracles in a vacuum. They require other nutritional factors to do their jobs. If one has an ongoing deficiency, it can take a while for the body to build it's "B-vitamin handling system" up again.

People use B vitamins for different reasons. The result you see depends partly on the how absorbable your supplement is, and on how your body is able to use the vitamins. This is an individual thing. I'd give it at least 6 weeks.

Unless you have a B12 deficiency, in which case it may take longer. With B12 defects you want to get the methylcobalamin form, and at least 1 mg (1000 mcg) per day, preferrably sublingually as many people do not absorb this one from the gut very well.

Best wishes.