My son went to private elite college and now works as an intern at a low wage. How long should I support him?

As needed. Internships are a way to learn new skills and get an edge on permanent employment. They currently are exploitative and largely unfair because they can be held mostly by those with the resources to accept no or low pay. But since the advantages are the 'edge' in employment, they are a good idea. Support you child as needed and as your resources permit until he/she is on their own.