You are eligible to receive the credit for the first 4 years of post-secondary school.
The American Opportunity Tax Credit modifies the existing Hope Credit. The AOTC makes the Hope Credit available to a broader range of taxpayers, including many with higher incomes and those who owe no tax. It also adds required course materials to the list of qualifying expenses and allows the credit to be claimed for four post-secondary education years instead of two.
https://www.irs.gov/uac/american-opportunity-tax-credit
