We have just released a new version of the MangoNoSQL module that has finer grained purging. So basically you can now accurately purge down to a one day level. This setting is not enabled by default for periods larger than about 25 days but can be enabled by checking the Purge within shards checkbox on the System settings page in the Mango NoSQL Information area.
Excerpt from help file:
Purge within shards
This setting governs if the purge task will delete values from within a shard file. With this unchecked, purges are much faster because data is purged in whole shards as opposed to excising individual samples. If the purge period is smaller than a single shard (about 25 days), however, the data older than the purge configuration is deleted from all shards, regardless of this setting.