Firstly ask yourself if the cereal is doing any good at all, because there are potential risks. Current studies show thickened feeds don't work.
I would wait about 10-15 minutes then offer a small amount.
However if your baby is happy and growing properly medicating reflux is NOT worth the risk. Also take an oz of formula and drop it on the floor from shoulder height, you might be surprised how much an oz really is.
Baby cereal, added to thicken breastmilk or formula, has been used as a treatment for GER for many years, but its use is controversial.
Does it work? Thickened feeds can reduce spitting up, but studies have not shown a decrease in reflux index scores (i.e., the “silent reflux” is still present). Per Donna Secker, MS, RD in Gastroesophageal Reflux Disease PDF, "The effect of thickened feedings may be more cosmetic (decreased regurgitation and increased postprandial sleeping) than beneficial." Thickened feeds have been associated with increased coughing after feedings, and may also decrease gastric emptying time and increase reflux episodes and aspiration. Note that rice cereal will not effectively thicken breastmilk due to the amylase (an enzyme that digests carbohydrates) naturally present in the breastmilk.
Is it healthy for baby? If you do thicken feeds, monitor baby’s intake since baby may take in less milk overall and thus decrease overall nutrient intake. There are a number of reasons to avoid introducing cereal and other solids early. There is evidence that the introduction of rice or gluten-containing cereals before 3 months of age increases baby's risk for type I diabetes. In addition, babies with GERD are more likely to need all their defenses against allergies, respiratory infections and ear infections – but studies show that early introduction of solids increases baby’s risk for all of these conditions.