Sorry I misread your initial request.
To go back a step, currently there is no visitDuration dimensions like
pageDepth. This is an open feature request. Although once you have
this, it will only be able to be combined with a very limited amount
of data, like visits.
Requesting timeOnSite returns the total time for example total time
spend by people from Google.
Filtering by timeOnSite returns a subset of data across all visits.
(ie total time spent for each entry returned)
If you want to select a subset of visits, and get all the metrics
associated with those visits, then you'll need advanced segmentation,
which is currently not avaliable through the API and an open feature
request.
Hope this clears things up,
-Nick