The best representation for duration between dates in rest api

Question

I'm implementing a rest API to repent some business stuff. One of the elements to return is the duration of some operations, represented by a specific time. I am wondering: what are the most common ways to represent that kind of data? Currently, I am trying to use ISO 8601 duration, but maybe this is not the best format for some reasons?

{
  ...
  "averageDuration": "PT9.705S"
  ...
}