Unless they've changed their policy in the last year, it should be added to the end of your current furthest expiry date on any single plugin.
That is why, if you add a new product (say they release a new Abbey Toad plug) it is auto WUP'd for on year. Now, if that product WUP if further out than all your other plugs, and you re-WUP all your products they will be WUP'd to that new plugin's date plus one year.
I've done this for the last three Abbey Road additions since they require purchase where new Mercury plugs are added free of charge and *do not* change my Mercury WUP end date.