Up until at least 3 years ago, accounts were considered dormant if untouched for 48 months; deletion would occur sometime thereafter: How to Prevent Miles Expiration?

While the current rules don't appear to delete or close accounts, it is not really in your best interests to allow an account to go undisturbed for a long time. At the least, you'll forget about it and not gain benefit from the miles; at worst, under some future rule change, the entire account and all its miles will be deleted. Use up your miles as best as you can if your A3 account isn't something you find yourself in much need of anymore.