On average, each iteration of certification takes 5 - 10 business days. Some factors such as setup preparation and the integration configuration can influence the duration of certification iterations. Certification review could easily take longer than 4 weeks.
Other factors that influence the process are as follows:
Number of iterations required: The certification team might go through multiple iterations (a maximum of 3 iterations). It's mostly due to finding issues or bugs in the integration that need partners attention before completing the certification.
Quality of the submitted artifacts: The quality of the submitted supporting documentation, software installer package, or any other artifacts could cause delays in the review process and extend the duration of certification iterations.
Complexity of integrated features: The complexity of the integration and its features can also influence the time it takes for the certification team to review the integration.