You can't set a discrete value for duration, it's limited to short|long (7 or 25 seconds respectively).

By using ShowToastXml this (and all other) elements can already be set. I'll expose 'duration' and 'scenario' to ShowToastJson, but since neither of these are recommended for use (ref) and to keep ShowToast simple, if you want to use any custom attributes use ShowToastXml or ShowToastJson.