That command normally take 2 NSUP cycles, one to purge and another to verify. By default NSUP is configured to run once every 120 seconds, but depending on the amount of data in the namespace and underlying hardware a cycle may take longer than the default period so you would need to look at you logs to see how long an NSUP cycle normally takes.
That said, this command is also obsolete! If you are running a build newer than 3.12 then you should now use truncate: https://www.aerospike.com/docs/operations/manage/sets/index.html#truncating-a-set-in-a-namespace. For the application, the effect of truncate is immediate - you will not be able to read the truncated data and new writes will not merge with the truncated data. Operationally it does take some time to drop all the records from the index (basically the time to run a meta data only scan). Additionally, in Aerospike Enterprise, truncate is durable through a coldstart.