It usually takes 3 days before you notice a difference. If you've got a viral infection, the antibiotics aren't going to do anything for it- they'll just help keep you from getting a bacterial infection as well.
Fluids. Rest. Vitamin C. lather, rinse, repeat.

Remember, if you don't treat a cold, it will last about 14 days. If they give you something for it, it'll last about 2 weeks.