Estimating time to author software help manual

[Jennifer C. Bennett]

Hello,

I've been researching ways to estimate the time it will take to write a 
software user manual.  I thought I'd ask this question here on TECHWR_L to 
see what others have done to estimate time.  What kind of questions should I 
ask to help me make an estimate?  If I were to ask for screenshots of each 
screen, menu and toolbar, how could I use this information to come up with a 
reasonable estimate?  I figure this information might give me an idea of the 
complexity of the application, but I don't know how I would translate that 
to a ballpark figure of hours it would take to write the documentation and 
create graphics.

I asked the person I may be working with how long they think the user manual 
will be and they said they figured it would be 70-80% of a competitor's user 
manual.  I did the calculation and 80% came out to 105 pages.

I will be using Word, which I am very familiar with.  I will also be using 
Snag-It 8 to create detailed graphics - screenshots edited to show small 
sections of the screen with callouts, rather than just putting in one 
screenshot of the screen where the text is too tiny to read.  I love making 
these; they seem to have been much more helpful to users that the 
full-screen screenshots I used to make, but they are time-consuming to 
create.

I look forward to reading how others have solved the estimating issue.