Currently, the device class duration seems to support only numerical sensors that have a unit of measurement of h, m or s. It would be great if text sensors containing ISO-8601 strings (e.g., PT2H15M38S for 2 hours, 15 minutes and 38 seconds) could be supported for device class duration.
As the device class timestamp already supports ISO-8601 timestamp strings, supporting ISO-8601 duration strings would be a nice enhancement for time spans.