The ideal frequency is something you'll have to work out for yourself. The level of activity of your audience is something to consider. After that, you must strike a balance that is acceptable to the audience. You don't want people to unfollow you because you've flooded their newsfeeds. You also don't want people to wonder if you've vanished from the face of the Earth. There are certain parameters, but the ideal frequency can only be discovered by trial and error.