What is the proper amount of time to allow for the Ro-Tap sieve shot size test? Some specifications say 5 minutes. Is this always the best practice?

Several shot peening specification do state the the Ro-Tap (rotating and tapping machine used with nested sieves for particle size analysis) shall be conducted for 5 minutes. However, you can verify if this is appropriate by performing the analysis at the recommened 5 minute time and then, using the same 100 gram sample of media, perform the analysis using 6 minutes. If the media amount captured on any sieve varies by 1% or more then continue to increase the sieving time by one minute increments until you do not have 1% or more variance.