@rartych could it be also considered #137 into this PR? Think it makes sense

On general level it is covered as duration is listed in the registry:
https://spec.openapis.org/registry/format/duration.html

The open question is whether it should be mandated to use strings for duration or leave the decision to subprojects (to use integers or object types).

On general level it is covered as duration is listed in the registry: https://spec.openapis.org/registry/format/duration.html

The open question is whether it should be mandated to use strings for duration or leave the decision to subprojects (to use integers or object types).

I agree with @rartych that we still have an open question regarding whether we require duration to be always expressed and formatted as "format: duration", or whether we just allow subprojects to adopt this format, and in this case we recommend to add explicitly the format: duration, but we do not mandate this format to be adopted in all properties dealing with duration.

My feeling is that we still don't have enough consensus in issue #137 to require all APIs to adopt RFC 3339 format for next meta, as that measure would impact many APIs, breaking backward compatibility.

After review I agree with that.
We need to reach consensus on that.

From some documentation found in "open internet" ISO 8601 indicates for duration. So as initially using RFC3339 as a reference would fit in our purposes but let's see within Issue #137

In both basic and extended format the complete representation of the expression for duration shall be [PnnW] or [PnnYnnMnnDTnnHnnMnnS].
...

When	an	application	identifies	the	need	for	a	complete	representation	of	a	time	interval	through	its	duration	and	context	information,	with	duration	in	the	format	with	designators,	it	shall	use	an	expression	in	accordance	with	4.4.2	using	a	complete	duration	representation	as	defined	in	4.4.3.2.	
      Basic	and	extended	format:	PnnYnnMnnDTnnHnnMnnS	
                                                        PnnW	
      Example	1:	P2Y10M15DT10H30M20S
      Example	2:	P6W

RFC 3339

Durations:

   dur-second        = 1*DIGIT "S"
   dur-minute        = 1*DIGIT "M" [dur-second]
   dur-hour          = 1*DIGIT "H" [dur-minute]
   dur-time          = "T" (dur-hour / dur-minute / dur-second)
   dur-day           = 1*DIGIT "D"
   dur-week          = 1*DIGIT "W"
   dur-month         = 1*DIGIT "M" [dur-day]
   dur-year          = 1*DIGIT "Y" [dur-month]
   dur-date          = (dur-day / dur-month / dur-year) [dur-time]

   duration          = "P" (dur-date / dur-time / dur-week)