Methylprednisolone - How Long Does it Take for Pain Relief to Begin?

What are you taking it for? You mention inflammation.

Methylprednisolone is a steroid. Steroids work in part by suppressing your body's immune system, and this reduces swelling and inflammation. It is used to treat many different inflammatory conditions such as arthritis, lupus, psoriasis, ulcerative colitis, allergic disorders, gland (endocrine) disorders, and conditions that affect the skin, eyes, lungs, stomach, nervous system, or blood cells.
Without knowing more about you it is difficult to give an accurate answer.
If you are taking Methylprednisolone for arthritis for example most patients usually start to feel the effects of steroids within a few days. Some patients will start feeling better hours after taking the first pill. You are best to ask your doctor what to expect from your treatment.