How long will it take to purge visitors from the database and keep 3 to 6 months of data?

The time it takes to purge records depends on the system speed as well as network speed (assuming database is on separate machine), but this process can still take a few hours to complete.

We recommend gradually deleting the records. Start by deleting records older than 2 year. Once this is complete, delete records older than 1 year 6 months and so on until you reach 6 months/3 months (based on the records you want to keep). This will keep the changes under a limit and also have reasonable time for each operation.