It depends on the item description.
Activating a magic item is a standard action unless the item description indicates otherwise. However, the casting time of a spell is the time required to activate the same power in an item, regardless of the type of magic item, unless the item description specifically states otherwise.
So if the ring description says "The wearer can activate Enlarge Person 1/day", it's a one round activation time. If the ring description says "The wearer can activate Enlarge Person 1/day as a standard action", it's a standard action activation time.
-Hyp.