I've been feeling low on energy lately so i took some b vitamins. how long will it take them to start working?

It depends on cause: There are a number of reasons why someone can have low energy- including, but not limited to, lack of sleep, poor nutrition, thyroid disease, depression, stress, anemia, other medical problems. It would be important to know the cause for low energy to determine what the appropriate treatment would be and how long it would take to be effective.