The classification of "over a period of time" regarding dust contact typically depends on the context in which the term is being used. For example:
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Occupational Health and Safety: In workplace environments, exposure limits are often defined by regulatory agencies such as OSHA (Occupational Safety and Health Administration) in the U.S. These limits usually pertain to average exposure over an 8-hour workday or a 40-hour work week.
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Environmental Guidelines: In terms of environmental exposure, "long-term" exposure could refer to a duration of months to years, especially in health studies assessing the effects of dust (e.g., from construction or mining).
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General Life Context: For day-to-day life, being exposed to dust for even a short time can produce some acute effects, while chronic conditions might develop over months or years of regular exposure.
If you're looking for specific definitions or standards, it would be helpful to know the context (e.g., workplace safety, environmental health, etc.) for a more precise answer.