The idea of purging is to remove all O2. Ideally, you figure the volume of your pipe in cubic feet and then divide by the flow rate converted to cfm, to get an approximate time needed in minutes to purge your pipe. But that does not take into account leaks etc. Usually argon is kept flowing a little into the area even after purging is taken place to keep the air from sucking in. You can test the purge with a "match test, open a piece of the pipe up and stick a match in. If it goes out quickly then its good.