How often should one post new content on his social networks?

It depends how much attraction he/she needs or want to make profile a crowd puller some people update every5 mins ( who has so much time available) so totally depends on individual.

It depends on which social network you are talking about, it it is personal or professional, i.e.: Facebook Vs. Linkedin

It depends on what you want adn how you do it, because if you over do posts you might end up being blocked or users may hide feeds from your profile or page.

There is no need for scheduling a time for posting new content , you can have many contents for a single post and this is depended on the post theme itself and how it could interest people for posting their comments ,