Resetting "check engine" light

1997 VW Jetta GL 2.0: 83000 miles

I recently had the check engine light looked at the dealer and the purge valve was malfunctioning and needed to be replaced. I replaced the purge valve, however the check engine light is still on. The mechanic told me that the check engine light will turn off after about 100 miles of driving, assuming I replaced the part correctly. This seems kind of strange to me, but is this true that the check engine light will automatically reset itself after a certain mileage?