Some people grind for few seconds, others grind for a few minutes.
Bed partners find the sound disturbing but the grinder is oblivious to the noise. They only awaken partially during the grinding and therefore, remain unaware that they are doing it in the first place. It depends on the person, but some people can have hundreds of teeth-grinding episodes during a night. Generally it is first detected by a dentist because it causes excessive wear of the teeth. Often the front teeth of a grinder begin to look flat and their enamel tends to be worn away.