A "month internship" is a short period of time, usually lasting a month, where someone works in a specific job or industry to gain experience and learn new skills. Full definition
This is a 3 month internship at # 18,000 with the view to move to # 20,000 up on successful internship period. (inspiringinterns.com)
A 4 - month internship in the middle of this intense 12 month professional program further ensures that students understand the marketplace and are able to integrate their academic knowledge with a workplace functionality. (sciencemag.org)
3 - month internship with management consultancy firm supporting small businesses with guidance on business management and policy development. (youthcentral.vic.gov.au)