I observe that the file walker takes around 12 minutes per terabyte of data, on a healthy non-SMR drive and ext4 defaults.
A good approximation for testing time of the file walker process is to run any tool to measure disk space used by the storage directory, e.g. du. It will roughly perform the same disk operations.
There is a suggestion to add some log entries that would indicate when the file walker process starts and ends. But so far, this is all just approximate.