Purview Scanning Benchmarks

Typically depending on the scan level, what would affect this process is the data source, volume and complexity of the data structures. You can adopt best practices such as tuning the scan levels and scope definition to ensure you limit to necessary subsets of data to reduce processing time and resource consumption. Microsoft provides some helpful information on best practices on this document (Best practices for scanning data sources in Microsoft Purview | Microsoft Learn)which I encourage you to review.