You are ready once the ISMS exists on paper and has started to leave a trail in practice.
On paper, four things should be finished and approved: the scope statement, the risk assessment with its treatment plan, the Statement of Applicability, and the policies and procedures. Everything else refers back to these. If the risk assessment is still a draft, the auditor has no baseline to test the controls against.
In practice, the controls need to have run long enough to produce records. A few weeks is the minimum, and two to three months is better. An access review held once, three onboarding tickets with completed security training, a backup restore test with its result, a supplier assessment for your cloud provider: these are the records an auditor samples. A system set up two weeks ago leaves almost nothing to sample, and the audit turns into a document review.
Certification bodies also expect the ISMS to have run long enough to leave evidence before Stage 2, and three months is the usual rule of thumb. The internal audit and management review both have to fit into that window. A company that finished its documentation in January could audit in March, hold the management review in April and go to Stage 1 in May.
If you are unsure, a gap analysis before the internal audit shows what is still missing without producing formal findings.