Apply the Metrics and Fudge Factors
Now that you’ve got some kind of page count, it’s time to figure out how long the document will take. When I talk about hours in this section, I mean hours. Many people estimate by eight-hour day, which includes time spent fixing your computer, e-mail time, chat time, and time spent dealing with other minor emergencies.
One other note: I have found that estimating the hours per page works best on manuals over sixty pages long. With shorter documents, one bad snag can set your hours off significantly. For example, I had eight-page documents that took anywhere from 20 to 80 hours to complete.
Estimating Documentation Projects