There is no straightforward answer to this question as it depends on the size and complexity of your organisation, what systems and processes are already in place and what resources are available.
However, in URM’s experience it typically takes between 6 and 9 months for a small, low complexity organisation to fully implement ISO 27001.
With larger, more complex environments, 9 to 18 months is closer to the norm for fully establishing an ISMS. This naturally assumes that the appropriate resources are made available to achieve the desired outcomes.

ISO 27001 Clause 4.2, Understanding the Needs and Expectations of Interested Parties
URM's blog examines ISO 27001:2022 Clause 4.2, covering interested parties, their requirements and how these are addressed through the ISMS.
URM assisted over 500 organisations achieve ISO 27001 certification, here are the critical steps when implementing an effective information security system.
URM's blog explores ISO 27001 Clause 4.1 & how to identify organisational context, assess internal/external issues, and support effective ISMS decision-making.
URM’s blog explains how to plan and execute effective and conformant internal audits of management systems at each stage of the internal audit process.

