The amount of time it will take you to write your book depends, broadly, on two things:

1. How clear you are on what you want to write about.
2. How much research you have to do, as opposed to writing about something you already know about in great detail. Research takes a long time.

The late W Richard Stevens (my hero) wrote TCP/IP Illustrated (Volume 1, The Protocols). This has which has about 600 pages. Stevens said it took him about an hour per page, so 600 pages would take about 75 8-hour days, or 15 normal working weeks.

When I wrote Practical TCP/IP which has about 800 pages, it took about 3 hours per page, giving a total of about 300 8-hour days, or 60 weeks.

Time to draw diagrams

You need to consider how long your diagrams and illustrations will take, too. We reckon an average of about half an hour per diagram. When I wrote The Joy of X, drawing the diagrams by hand, on paper, took all morning, every weekday, on a three-week holiday in Germany. That's about 50 hours. The book has about 350 pages , with say, a diagram on every second page -- a total of about 175 diagrams, or about 20 minutes per diagram. That looks about right, as these were hand-drawn, so I didn't have any re-drawing or fidgety alignments and improvements to worry about.

Time for screenshots

If you have a streamlined way to: capture screenshots; save them; and name them using some disciplined, easily-remembered naming convention, then each screenshot should take less than five minutes. (Most of the time is spent arranging the screen so it has everything you want to illustrate, and in the correct position.)