Well, if this is a theoretical situation, and I have lots of free time....around 20 hours.
The first few hours are spent putting in backgrounds for the mission (I always start there first before moving on. The backgrounds set the environment for me to work in, so I end up spending 2-3 hours on getting them right), after which comes ship placement and the initial few messages (just to get the mood right). After this I let the mission 'grow', as it were, into what was planned, addings the events and messages as I go along, only adding new ships in once the events I was just working on are completed and well-tested.
Eventually, it becomes a mission. Playtesting is one of the worst parts of it tho, cause once I get out of the mission I usually remember all the big problems and forget about the tiny problems, and usually end up playing through a few extra times just to play those....its a pity that an events file can't just be dumped onto the HD stating what all happened and if everything was correct, at least we could refer back and correct it rather than playtesting all over again.....