A German study went about to determine the answer to just that question... They prospectively followed 760 children born in 1990 and followed them for 12 years. Parents recorded the child’s illnesses in a diary and answered structured questions yearly up to age 12.
This is what they found... The mean number of infections per year for each age group (standard deviation):
• 0-2 years: 3.4 episodes (3.7) per year
• 3-5 years: 2.3 episodes (2.6) per year
• 6-12 years: 1.1 episodes (1.2) per year
Two standard deviations above the mean can be still considered within normal limits, though on the high side of normal:
How many respiratory infections in children are "normal" per year? Allergy Notes by Dr. Ves.