The licence is valid for Office 2016.
If I remember Microsoft's licensing model correctly, you can use Office 2016 & all updates for Office 2016 for the rest of your life. But only for Office 2016, you will not get newer versions or upgrades (e.g. Office 2016 -> Office 2019).
Unlimited is a matter of definition ;)
It is not a subscription that you have to renew, i.e. it never expires.
However, since online activation is necessary in the case of a new installation and Microsoft will probably discontinue the activation service at some point, it is still not completely unlimited. But it will probably take several years (I estimate at least 10) before the activation service is discontinued.