If I have compaction enabled, like SizeTieredCompaction, my SSTables get compacted until a certain size level is reached. When I "delete" an old entry which is in an SSTable partition that is quite old and wont be compacted again in the near future, when is the deletion taking place?
Imagine you delete 100 entries and all are part of a really old SSTable that was compacted several times, has no hot data and is already quite big. It will take ages until it's compacted again and tombstones are removed, right?
When the tombstone is merged with the data in a compaction the data will be deleted from disk. When that happens depends on the rate new data is being added and your compaction strategy. The tombstones are not purged until after gc_grace_seconds to prevent data resurrection (make sure repairs complete within this period of time).
If you override or delete data a lot and not ok with a lot of obsolete data on disk you should probably use LeveledCompactionStrategy instead (I would recommend always defaulting to LCS if using ssds). It can take a long time for the largest sstables to get compacted if using STCS. STCS is more for constantly appending data (like logs or events). If the entries expire over time and you rely heavily on TTLs you will probably want to use the timed window strategy.