Trev,
The default hours are set under 'Tools\options\Calendar\hours per day'.
This must be a value as a task by definition must take some period of time
and hence some work. Project uses this value to calculate the end date for a
task so it must have a val;ue greater thasn '0'.
A '0' duration task is merely a milestone indicating that a particular point
in the project has been reached, it is not really a task as such. If
management need to see a task where an estimated duration has been given then
leave the "New Tasks have estimated durations" ticked under the schedule tab
on 'Tools\Options'. This places a question mark against the default duration
for the task. If you set all durations on your tasks to zero, where the
duration is unknown then the project will show an unrealistic overall
timeframe. When assigning a task it must be possible at the time to
guesstimate a realistic duration?