An organization must consider a number of factors when selecting a time for the assessment, the primary among them being: "Is the organization ready for an assessment?"
An assessment should also take place at a relatively unintrusive time. There probably is never an ideal time for an assessment, but certainly the weeks before a delivery to a customer is not a good time.
For follow-on assessments, the organization needs to consider whether enough time has elapsed since putting in place improved processes needed or recommended by the last assessment. Scheduling a second assessment without allowing time for improved practices to be implemented tends to be extremely frustrating for the assessment team and the organization. One rule of thumb is that an organization needs to demonstrate that the improved processes have been in place (and practiced on at least one project) for at least six months.
Many organizations have found that alternating health checks and assessments on a yearly basis works well. A yearly check on progress helps organizations maintain a focus on quality.
Some companies have found an Assessment Readiness Survey useful in helping them answer this question (see "Assessment Readiness" at the end of When Is an Organization Ready for an Assessment?).