This appears to be the command to set the default meeting length for iCal events:
defaults write com.apple.iCal 'Default duration in minutes for new event' X
Where X is 30 means that the default meeting length is 30 mins.
My question is - what is the equivalent of this API on iOS? I haven't been able to find it in the API docs - but perhaps I'm not looking in the right place.
That's not possible on iOS, at least not system wide. You can provide your own UI in your own app which has a different default length.