Note: Not a solution but marked as such to elevate a key reply
For all Business, Advanced & Enterprise users (who have the Advanced Editor for Formulas), there is a workaround for you! 
For all your projects that you are using Asana’s native Estimated or Actual hours fields, do the following for either field:
The examples below are for Estimated time, but you can follow exactly the same for Actual time field also.
To get results in hh:mm
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Create a new Formula custom field and call it “Estimated time (hh:mm)” (or whatever you want)
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Enable the Advanced Editor button and simply insert the field ‘Estimated time’ all by itself and save the formula field, as per below:
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Make sure to add the field to your library (first checkbox) and click save.
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Go to your dashboard and create a chart based on this new formula field “Estimated time (hh:mm)” which will display your hours in hh:mm instead of weeks/days!
To get results in hours (with decimals)
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Create a new Formula custom field and call it “Estimated time (h)” (or whatever you want)
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Enable the Advanced Editor button and simply insert this formula: (Estimated time +0)/60 and save the formula field, as per below:
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Make sure to add the field to your library (first checkbox) and click save.
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Go to your dashboard and create a chart based on this new formula field “Estimated time (h)” which will display your hours as a number with decimals instead of weeks/days!
In both cases, in your list view, use the Hide button to hide this new formula field from your list view since it is simply used to translate the values into your dashboard - no need to have duplicates in your List view. 
Kudos to @Tetsuo_Kawakami for bringing this to light here!