When planning a project, in which field should I put the amount of work required for a task? In other words if building a wall takes 10 man-days, where do I put this 10?

I have so far been putting it in the Duration column, but every time I update the project, MSP modifies the Duration column to reflect the new duration of the project, including delays etc. This loses track of the original duration, so this can't be right.

What does Duration mean? The number of planned elapsed days (including holidays and non-working days, or not?), the number of planned man-days, the number of actual elapsed days so far, or the number of actual man-days so far, or what?

And when updating the project, where do I put the actual % complete of each task at this moment in time? I have so far been using the % Complete column (seems logical, right?) but I'm not sure this is right, because MSP is sometimes preventing me from updating this field.

Thanks - Rowan