The D is subtle. One notices no more colds/flu, that is one big thing. If you have pain, it might diminish some. People with neuropathy may see less pain with it...(one study done on diabetic neuropathy).
According to Dr. Oz (if one is to believe that), Vit D encourages the destruction of cancerous cells in tissues, before they can grow. It is not possible to see or feel that, but the long term prognosis is less potenial cancer. I myself have not read that study, but he mentioned that on his show once.

The B12 may start working in 2 to 3 months. If you have some nerve repair starting, you will notice that. Tingling or pain may return briefly to previously numb parts.

Most people get a burst of energy with improving B12. It will improve peripheral nerve functions, but that is slower than the energy part. I never get the energy part, but the methylB12 in particular helps with melatonin manufacture, and hence better sleeping. I did get that effect during my menopause.