It probably means what it says; Until the next combat round.
Full Defense is an "Anytime" action that you may take out of order (think interrupt action from SR5).
If you use it against an attack at the beginning of the combat round then it will last for the remaining of the combat round which in this case is also equal to the entire combat round.
If you use it against an attack at the end of the combat round (granted you still have a major action to spare) then it will also last for the remaining of the combat round which in this case might only be that single attack (in this case maybe you should consider using one of the other "Anytime" actions that only work against a single attack, such as Dodge, Block or the Hit the Dirt action (which you can also combine with the Tactical Roll edge action)).
