4 Months Old. Most spitting up resolves between 4-6 months of age. Many factors contribute to the improvement, including the normal maturation of the valve between the esophagus and the stomach, the introduction of solid foods, and the baby starting to sit up which makes it more difficult for milk to come up and out.
Depends Upon Cause. It depends upon why. If it due to an immature esophageal sphincter muscle ( reflux), it can be early at 3-4 months. But often seems to coincide with standing up and cruising as those muscles appear to mature at the same time. If due to a formula intolerance, not until it is changed. And due to overfeeding, when the volume is decreased. This assumes no anatomical defect.