I never want ESO+ for more than a month at a time, so I always buy it, check it's active and then cancel it immediately and I always get the full 30 days, it just doesn't renew when it's finished.
SeaGtGruff wrote: »If you plan to continue playing ESO after canceling ESO Plus, make sure you cancel it through the proper channels.
There have been players who apparently canceled it through their credit card companies or banks and it resulted in the bank reversing the charge for their current subscription rather than stopping an upcoming renewal of their subscription, resulting in the total loss of their ESO account.
There is a link at the bottom of forum pages which says "Cancel ESO Plus," so presumably that would be the preferred way.