My advice is to see if you can charge for the project, as Jody says. How are you ever going to charge by the hour? That just doesn't work for writing, and you'll probably end up getting way too little.
I don't know about you, but I sometimes spend quite a bit of time thinking about a project, running down research leads, rewriting, rewording, etc. It's hard to explain to all that to a potential employer. The actual time you put in doing the writing is usually the least of it.