Pinched nerve in neck going on 48hrs. Only left side in neck, shoulder, arm. Arm is also partially asleep/numb feeling. When to see a Dr?!

NOW! This suggests a compression neuritis or disc issues. I suggest an immediate examination and a neurology consultation-ASAP!
See doc soon . Since these symptoms have not resolved and are worsening, a good exam should determine which diagnosis is correct and which treatment is needed to fix this problem .
Be seen. I would be seen as soon as possible, just to make sure it will not cause you any long term issues.
History. Did you have a traumatic event to cause this? What prompts you to ask currently? If this is new, you should be seen very promptly to be evaluated.