I have low energy and my doc gave me a vit b12 shot today. how long should it take for me to feel an increase in energy?

You have assumed that Vit. B 12 deficiency is making you feel weak but I doubt that. You have not given enough information to actually figure out what may be causing the weakness but it is rarely Vit. B12 deficiency. Are you drinking enough water and eating nutritious food including micronutrients? You would be surprised that simply exercising may actually make you more energized than before.

It is not feasible to provide a meaningful opinion without taking additional history, physical examination and may be some tests. It would be prudent to see your doctor and discuss if you may be depressed. Counter-intuitive as it may seem, exercise may help. Start with low intensity exercise such as walking and gradually increase the intensity as tolerated. Drink more water. Wish you good health!