org-time-clocksum-fractional-format

org-time-clocksum-fractional-format is a variable defined in `org.el'.
Its value is

"%.2f"

Documentation:
The format string used when creating CLOCKSUM lines,
or when Org mode generates a time duration, if
`org-time-clocksum-use-fractional' is enabled.

The value can be a single format string containing one
%-sequence, which will be filled with the number of hours as
a float.

Alternatively, the value can be a plist associating any of the
keys :years, :months, :weeks, :days, :hours or :minutes with
a format string. The time duration is formatted using the
largest time unit which gives a non-zero integer part. If all
specified formats have zero integer part, the smallest time unit
is used.

You can customize this variable.