[engineer at cockroach]

How high you can set it depends on your access patterns -- there is some overhead to iterating though MVCC revisions during reads, in addition to the on-disk space you mention.

If your workload involves frequent writes to the same rows, GCing some of those revisions sooner would have a greater impact, whereas if you have a write-light workload, or if your writes are spread over rows such that a given row doesn't doesn't see frequent repeated updates, then you could probably use much a higher GC threshold with minimal overhead.