Co-op requires all players to have PS+ in order to play. Furthermore, all Share Play sessions, co-op or otherwise, are capped at a length of one hour. Once you hit the limit though, you're free to start another session with the same members immediately, as many times as you like. This is, of course, to prevent people from setting up a Share Play session for a friend, and walking away to let them play the game in its entirety without purchasing it.