As you have probably found with google there are no accepted standards as to what makes a painting a plein air piece. To keep oneself from becoming a prevaricator it would be wise to work on it at least a portion of the piece on location, outdoors. Beyond that it is up to the artist on how much studio work they want to do on it.
However, when it comes to shows and events, most of them have a set of rules to be followed and some are very strict saying it must be 100% done on location, others less so. The least restrictive I have looked at participating in allows a max of 25% studio work. They don't specify if that is per square inch or a time related percentage.
I heard a story from several reputable sources about a fairly well known plein air event less than 4 years ago that didn't have a very strict set of rules and was not monitored. According to those telling the tale, one of the participants who is a relatively famous plein air painter, spent almost the ENTIRE TIME in their motel room. None of the artists there ever saw them leave their room for the week long event, nor were they spotted painting en plein air. Their car sat in the parking lot for most of the time. They won some awards and sold a bunch of paintings.
Nobody chose to rat them out.
This thread of mine on WC was created when I had similar questions to yours but more related to shows and events.