Count rows less than column value

You are close. Assuming you are filtering on your duration column in a visual, this should give you what you want. 

=CALCULATE( COUNTROWS( Table ), FILTER( ALL(Table), Table[Duration] <= max(Table[Duration] )))

The addition of the ALL function removes all filters in your visualisation

the addition of the MAX function (or any aggregator for that matter) will detect the current filter context and pass that to your calculate formula. 

Hence the filter function first removes all filters and then reapplies a filter on the Table[Duration] column to be less than or equal to he maximum value in the current filter context.