How often can I expect GC to run?

[Alexey]

@Climbingkid

If I understand this correctly then, I have 75% of my allocated storage taken up with data I am not getting paid for

This could be a case only when the missed data would be backfilled from the satellites side (US1 and SLC in particular), and when it happen you would still have a discrepancy between the physically used space on the disk (not on the dashboard) and a usage reported by satellites (Average Disk Used Space This Month).

The discrepancy between the used space on the dashboard (piechart) and the usage on the disk meaning that your databases are not updated with the correct usage. This can be fixed by enabling a scan on startup (it’s enabled by default) and restarting the node. You need to wait until used-space-filewalker will be successfully completed for all trusted satellites.
You also need to remove data of the untrusted satellites, if you still have it:

How To Forget Untrusted Satellites

Following the announcement to decommission Europe-north-1 and US2 satellites, manually deleting the remaining data of these satellites from the node can be a tedious task as it requires finding the blobs folder name for each satellite, so we wanted to improve the storagenode CLI to semi-automate cleaning up a satellite data after it is marked as untrusted. The storagenode CLI (versions from 1.88.0) provides a forget-satellite subcommand to remove satellites from the trust cache and clean up the…

@Climbingkid

I think if the plan is to move to short TTL data then GC has to be more frequent

There is no such a plan. TTL was here since the alpha. You always will have data with TTL or without, they are not a mutual exclusive.
For example, if you deleted databases, the info about TTL will be lost, so this data then will be collected by GC.