The time will depend on many factors including the server, the workstation and the communication between the two. I generally recommend running the purge on the server. Alternatively you can do it on a copy of the database on the local workstation, then move the database back to the server. 

You should run database verification - look for errors before running the purge, as errors in the database may impact the purge process.

Note that Timeslips v2018 is based on a different database and the capacities have been substantially increased. You might benefit from upgrading so that you would not have to purge data from the slip table, as well as being on a current version.