Hi Nicola,
I think that you can safely encode duration as <extent>. I'd encourage you to look at the <physdescstructured> element in the forthcoming EAD3. In it there are separate elements for <quantity> and <unittype>, which improves the data model from <extent> in EAD 2002. Further, on <physdescstructured> the @physdescstructuredtype attribute may be set to one of four options: spaceoccupied, carrier, materialtype, or otherphysdescstructuredtype. The last was included based on feedback from members of the community who asked for ways to use <physdescstructured> for duration and other data points related to audiovisual materials. The following might be an example:
<physdescstructured physdescstructuredtype="otherphysdescstructuredtype"
otherphysdescstructuredtype="duration">
<quantity>5</quantity>
<unittype>minutes</quantity>
</physdescstructured>
FYI - EAD3 was submitted to the Society of American Archivists' Standards Committee last week and I am hopeful that it will be accepted and published by the end of the summer.
Best,
Mike Rush
TS-EAD co-chair