When cost and usage data becomes available in Cost Management, it gets retained for at least seven years. Only the last 13 months are available from the Azure portal under Cost Management portal under
Exports. It's also available in the
Cost Details API. To retrieve historical data that is older than 13 months, use the
Exports REST API.
This is a suggestion for your internal planning: consider running a script every six months to update Cost Management, especially if you're looking for data beyond the past 13 months.