SSMA automates conversion of most statement. There are a few features and syntax which SSMA is unable to migrate. In such situation, SSMA issues a migration error and for each error, SSMA provides estimated manual conversion time. I often asked how do we come up with the time.
Those manual estimation time was calculated based on the actual average time it takes by our developer team to fix the issue (exclude testing time). The manual estimation is intended to help you further quantify the complexity of the issue to help planning the database migration.
However, you should be aware of the assumption and limitation of the manual conversion time estimate:
In addition, the manual estimate hours depends on the skills of resources performing the migration. In order to resolve the issue, you will need to understand the original Oracle source and understand how best to redesign the statement in SQL Server. Thus, you need resource with knowledge of both Oracle and SQL Server. If you do not have resource with knowledge in both technologies, then you need to have separate resources (Oracle DBA and SQL Server developer) collaborating to resolve the issue. In this case, you need to factor the number of resources to your project planning.
I often find customers use the estimated manual conversion time as a comparative number to rank complexity between one schema/database to another.
The manual conversion time can still be useful for rough order of magnitude (ROM) estimation, but for more accurate project costing and time estimate, it is best to have the actual person(s) performing the migration to review the error list carefully, consider the consider the design approach and skill level, then refine the estimate accordingly.