I've trawled the web for info on iCloud Photo Library's "Optimize Phone Storage" option on iOS, but can't figure out the exact logic applied.
iCloud Photo Library will automatically manage the size of your library on your device, so you can make the most of your device's storage and access more photos than ever
I assume the device will cache high-res photos up to a point, but what is this point in terms of MB remaining on the device?
In addition, does iCloud Photo Library perform any housekeeping on that cache, e.g. clearing out high-res copies you've not viewed in n days - if so what is n?