How long will this cold last?

Unfortunately there is nothing that can cure the common cold, you can try to make her more comfortable by turning on the hot water in the bath tub and shut the door (this should create a steam room, you are obviously going to hold your baby, NOT put her in the water, the water is only for the purpose of creating steam), sit with your baby in the bathroom for about 10 minutes. The steam should loosen up the mucus, then suction her nose with a bulb syringe. Also use you cool mist humidifier. They say that colds could last up to two weeks.