The purge valve is normally closed at idle unless the intake air temp gets above around 40deg C (if I remember correctly). The duty cycle when openned varies by the conditions.

One of the ways that you get a large leak code is when the purge valve doesn't open. The evap monitor test closes the vent valve (sealing the charcoal cannister), opens the purge valve until a ceratin vacuum is registered in the tank by the tank pressure sensor or a certain time has elapsed. Once the pressure is achieved the purge valve is closed and the sealed system is monitored for a period of time. A large leak is determined if the tank doesn't reach the required vacuum and a small leak is registerd if it doesn't hold for the specified time.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician