Does a ring require an “activate a magic item” action? Also, how frequently can one use a ring?
A ring is command activated (see page 192 in the DUNGEON MASTER’s Guide), unless the ring’s description says otherwise. Any such item requires a standard action to activate.
Unless otherwise stated in the ring’s description, there are no limits on the number of times it can be used or on how frequently it can be used. Since activating a ring requires an action, it usually cannot be activated more than once a round. Note that most rings function continuously once activated, which allows for virtually unlimited use unless the ring produces an effect that can be broken.