Duration of the ISO 20000 audit

Answer:
Duration of an ISO 20000 audit is agreed between the organization and the certificating company. It takes from e.g. one day (for small organizations) up to few days (in bigger companies). So, what you should do is pick-up a certifying company and agree with them length of certification audit.
Find out about certification in the article "Process to obtain ISO/IEC 20000 certification: Companies and individuals" https://advisera.com/20000academy/knowledgebase/iso-20000-certification-the-process-of-obtaining-a-certifica/
As for yourself, you can check your compliance to ISO 20000 using "ISO 20000 Gap Analysis Tool" https://advisera.com/20000academy/itil-iso-20000-tools/iso-20000-gap-analysis-tool/