How can I estimate the data volume that Nightfall needs to scan?

The total amount of data usage is not readily available in the Slack console. Nightfall recommends you export your Slack workspace data for historical scans on Slack. The exported archive file contains messages and links to files shared in Slack. Hence, the size of the exported archive file will likely be very small compared to the total amount of data to be scanned via Nightfall.

Nightfall recommends you export workspace data for up to the last three months, as an example, and request the Nightfall team to run a historical scan on this exported data. Once the historical scan is complete, Nightfall can help you identify the total data scanned (GB) for the respective time period. You can use this information to understand the average monthly data volume in your Slack environment to extrapolate the total data volume required to scan data over a larger time period.

For a basic understanding on usage limits in Slack, refer https://slack.com/intl/en-in/help/articles/115002422943-Usage-limits-for-free-workspaces