Duration of pain after a muscle biopsy should not exceed the duration of pain after treating a wound. The stitches keeping the skin attached are usually removed within 7 to 10 days. However, the tissue beneath might need more time to recover. The tissue has to form itself again. If the tissue is damaged too much, then scar is formed made of fibrous tissue.
This remodeling might take time and cause pain if it is putting pressure on the surrounding nerves. In the end, the pain is supposed to diminish or completely disappear. How long it would take depends on the wound and quality of the affected tissue (if there is a muscle disorder that was supposed to be detected with the biopsy then it might take longer), but usually you should notice changes in the intensity of the pain in two moths at most.