How long is the chickenpox vaccine effective for if you got it at age 5y?

Depends: This vaccine is a modified strain of the wild varicella virus & is thought to confer lifelong immunity in 85 -90 % of kids after 1 dose. A second dose was initially recommended only to teenagers at first, but now we generally want all kids to have at least two, to pick up the stragglers with poor initial response.Long term protection is a feature of other viral vaccines.

Life long: We now recommend a booster so that you should receive 2 doses to provide lifelong immunity. After one dose many people were still getting chicken pox infection, albeit mild.