Most of the work is simply gathering the documentation and evidence in a timely manner. After which it should not require more than a few hours a week complete the assessment process. Some variables which can impact the time required are: the size of the environment will have an impact upon the amount of time required to collect evidence requested, and if there are any external security frameworks that can be leveraged to support the assessment. Where external security frameworks are in place, and adequate supporting documentation can be provided, certification analysts can use these external assessments to satisfy a subset of Microsoft 365 controls, without you needing to supply additional evidence.