We use 6 mins as our purge time, I have also heard of 5 mins being used but never less. Being an operator and not an engineer, I was told and may have read in a design book, that boilers are designed and modeled as follows. After 1 min purge 90% of air has been replaced, hense after 2 mins another 90%. So after 2 mins 99.00% has been replaced. After 5 mins 99.999 % has been replaced, plus 1 minute for safety factor. Boiler design engineers would know for sure.