I was able to get ETA on the compaction operations running now by using nodetool compactionstats. But what I need is to know how long did the past compactions run. nodtool compactionhistory shows when the compactions are completed but time took by each compaction operation.
It depends upon the data and size of the sstables. But you may increase throughput to get it faster if your server is strong enough. Also, you check the compactionstats and see the total size and remaining size.