When a home air filter manufacturer recommends a filter change every
90 days, what duty cycle do they assume the filter wil be used?
1 hour per day?
10 hours per day?
24 hours per day?

I think they should list filter change recommendations in hours (e.g.
filter good for 300 hours of use).
(Some thermostats like my LUX have filter-life counters that show the
number of hours the filter was used since it was last changed).