org-table-duration-custom-format

org-table-duration-custom-format is a variable defined in `org-table.el'.
Its value is

hours

Documentation:
Format for the output of calc computations like $1+$2;t.
The default value is 'hours, and will output the results as a
number of hours. Other allowed values are 'seconds, 'minutes and
'days, and the output will be a fraction of seconds, minutes or
days.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.