Start with the documentation:
Instagram Developer Documentation[
^] which says:
Even though our access tokens do not specify an expiration time, your app should handle the case that either the user revokes access, or Instagram expires the token after some period of time. If the token is no longer valid, API responses will contain an "error_type=OAuthAccessTokenError". In this case you will need to re-authenticate the user to obtain a new valid token.
Which means that they don't normally expire at all, but you need to detect when they do and repeat your manual procedure to get a new one.