Here is the “Four-day Wait Rule” described in Super Baby Food:
Introduce only one new food at a time. After you introduce your baby to a new food, do not introduce another new food for at least four days. During the 4-day waiting period, watch carefully for signs of allergies.
Note: Some experts recommend a 3-day waiting period, some recommend waiting 5 days, and still others recommend a full week of waiting between new foods. Consult with your pediatrician and follow his recommendation.