How many hours should young people spend (minimum) on social media per day?
A PERSONAL recommendation is no MORE (maximum) than four hours.
Anxiety disorders induced by online social media activity are as varied and complex as any other anxiety. There is no single prescriptive exposure time that is valid for all people.
It is much more difficult to recommend a complete elimination of social media connections in today's world, much as denying any 'telephone' time was in the 1960's - 1970's. Too much of today's interactions, news and entertainment are simply there.
However, anything that becomes addictive (causes negative results in other important areas of life) or causes anxiety must be regulated in some way. My personal exposure level is based on a reasonable time to effect social interaction, some entertainment and/or news without letting it overwhelm the activities of a day. It is based on similar experiences with television exposure time.
Learning to manage personal time to get the most benefit from the limited "online" time is a good practice in any case. If more time is deemed necessary for connectivity (or if the limitation causes even more anxiety), then other methods will need to be used to address the anxiety in general rather than simply removing a source.
It is never the technology that causes the anxiety - it can only facilitate or possibly amplify an existing predilection for it. Any real anxiety issues need to be dealt with at a fundamental level with the individual.