My definition is the approximate/estimated distance that any significent percentage of the population under discussion would consider using a bicycle as their principal means to commute to/from their destination. I suppose if the population under consideration comprised only the members of cycling enthusiast discussion list the outside distance would be more than if the popualtion included the other 99.9% of the commuting population.

There will always be the outliers who cycle incredibly long distances just as their may be a few commuters who would use unicycle or highwheeler. Neither use would demonstrate that what is possible is necessarily considered practical by the vast majority of the population.