Optimal frequency for the amount of posts in several social media:
- Facebook: 2 posts/day max; 5-10 posts/week
- Twitter: at least 5 posts/day
- LinkedIn: 1 post/day; 20 posts/month
- Instagram: at least 1-2 posts/day
As for when, further research should be done for each social media used because you'll want to know the time when most of your target market are online and offline.
By knowing that, it will help a lot in determining when to post as you want to post on the exact time when it is the most efficient. You want to post during high user traffic with relatively low amount of posts. Therefore, further research is needed.