Do I need to be available every day?
No, you don’t need to be open to the public every day. But your location must always be able to receive incoming post and it should usually be processed within 24 hours after delivery.
It’s no problem if there’s occasionally a small delay, but regular mail handling is essential. For example, if you’re away for a full week and no one empties the mailbox or processes the post, that won’t work.