The only significant impact is that you cannot view workflow information for documents that had their workflow purged.
Therefore, you should only purge the workflow that has an age beyond the range that your users might view the workflow activities.
In general, specifying 30 days for the age parameter should be ok.
One more thing, if you have not purged the workflow data for long time (use the queries in the below discussion) then you should expect some performance issues while the request is running.
To avoid any negative effect on the performance of the system, run the purge at a time where user activity is minimum.
I just noticed that the referenced discussion is owned by you, too!
When you run this program, its always better to have a limit between 30- 90 days.
But the program deletes only the completed workflow. Just to be sure you can schedule in test environment .
Pl continue the discussion in your original post