ISO 27001 implementation duration depends primarily on the size of the organization:
- Companies of up to 20 employees – up to 3 months
- 20 to 50 employees – 3 to 5 months
- 50 to 200 employees – 5 to 8 months
- More than 200 employees – 8 to 20 months

Notes: This duration includes writing all the documents and performing all the activities. There are some other factors that influence the speed - if a company is using some tool or a consultant, if some documentation already exists, etc.

Examples:
- A SaaS company of 40 employees already implemented ISO 9001 and ISO 22301 - they might need less than 3 months to implement ISO 27001 because they already have lots of documentation and processes in place.
- A manufacturing company of 150 employees is using a consultant for implementing ISO 27001 - their implementation time will be closer to 5 months because of the external help they are getting.
- A healthcare organization of 500 employees is compliant with GDPR and HIPAA, and is using a documentation toolkit for ISO 27001 implementation - their implementation time will be less than a year.
- In general, for smaller organizations, the development of policies and procedures can take anywhere from a few weeks to a few months, because developing the necessary documents for ISO 27001 requires time for agreement, review, and approval by multiple stakeholders, and it takes time for employees to adapt to the new rules and change their activities accordingly.