"How long will it take to get to Sydney from Melbourne?"
To answer this you need to know three things:
* How fast are you going?
* What route are you going to take?
* How accurate are my estimates for speed and distance?
Whenever you estimate a task in terms of time you are joining three independent variables. The Productivity of the team, the Magnitude of the task and the Accuracy of your estimate.
* To drive direct to Sydney is about 1000 km.
Assuming you don't need to take any detours or get lost.
* The average velocity for the trip is around 100km/hour.
Assuming your car doesn't break down, or you have an accident, or the kids need the toilet every 20 mins ... etc.
So.. I could quote my estimate as 13 hours.
If I had done the trip a few time, and it always took between 10 and 13 hours then maybe I would be much happier to make that quote.
But... what if I was gambling $100,000 on being able to get someone to Sydney in that time.
I wouldn't be happy with the estimate any more. I would want to allocate extra time incase the unforseen happened.
What level of risk is your customer happy with? Have you asked?