How often does the Gregorian calendar coincide with China people's birthdays?

Four years. Birthdays of the lunar calendar and the solar calendar coincide every four years. This phenomenon is caused by the complete coincidence of the dates of the lunar calendar and the Gregorian calendar during the period from the first day of the lunar calendar to the New Year's Eve of the following year in one of these four years. If the first day of the first lunar month coincides with the Gregorian calendar 1 day in a certain year, then this person's lunar birthday coincides with the Gregorian calendar birthday.