Deduplication is one of the space efficiency features .
You can have a volume with only some of the space efficiency features on ; ( if you have a AFF device then by default all the features are enabled ; for FAS you need to enable space efficiency support.) .
Cross volume deduplication is supported only on volumes that are owned by nodes that are All-Flash optimized personality enabled. Inline deduplication is supported only for volumes that are either owned by nodes that are All-Flash optimized personality enabled or on hybrid aggregates.
You can enable compression out of the bat via vol efficiency modify ***** -compression true.
PS : on FAS volume efficiency is enabled means you are eligible ( but most of them have strings attached - manual work to enable )
Normal deduplication as post process can be achieved with :
There may have been a dedupe scan at one point and only 110 MB of data is still deduped, and the other 39+ TB isn't. Dedupe being off means a scan won't run. The data state is not impacted because that is a separate and more dangerous process to undo and not recommended without NetApp supervision.
I hope this helps.
Also on how long? It could be a while as you have spinning disks. Change rate will also come into play. If the data is static the initial scan may take hours or even up to days, but subsequent scans will be quicker.