Well there are gread differents between the projects. Some take 5 minutes like this one. Others take month. On my Worms game I worked about 4-5 month. So you can't say how long any scenario takes without knowing what the scenario should be about.

I am sure that the answer would really depend on the type of project; the interactions involved between different actors; the intricacies and nuances of the project; how well the project is to be designed; the speed, accuracy and fluency of the coder; and a big factor may be how long it takes to resolve any problems encountered.