I can give you an estimation based on the assumption that the typical school journey follows certain patterns:
Preschool: Many children start attending preschool at around age 3 or 4, depending on the country and individual circumstances. Preschool usually lasts for one or two years.
Primary/Elementary School: Children usually start primary/elementary school around age 5 or 6 and complete it around age 11 or 12. Primary/elementary school typically lasts for about 6 years.
Secondary/Middle School: After primary/elementary school, children usually enter secondary/middle school, which lasts for around 6 years, from ages 11 or 12 to 17 or 18.
High School: In some countries, secondary school and high school are combined, while in others, high school is a separate phase. High school typically lasts for 3 to 4 years, starting from around age 14 or 15 to 17 or 18.
Based on these common patterns, the estimated time spent in school during childhood (ages 1-18) could be around 12 to 14 years, or approximately 67% to 78% of childhood. Keep in mind that these figures can vary significantly depending on the educational system and individual circumstances in different countries and regions. For the most accurate and up-to-date information, it is best to refer to specific educational statistics and reports from relevant sources.