I've been working as a freelance editor for about a year and a half, and although I've worked for a number of different clients, I've never been as confident about how much time it should take to do something as I need to be.
There's often some wrinkle (borderline-unusable audio, a new style or direction the producer wants to go in partway through the edit) that slows the editing process. Conversely, there are many times when I finish my work in much less time than I had expected.
So: I have to put in a bid for what should be a straightforward job: edit 6 hours of footage--all scripted simple scenes--into four 35-minute pieces. There's a script with circled takes (not actual lined pages, but pretty close). It's very simple material. The intro/outro segments are Apple Motion templates that just need customization. The music for has already been chosen. Each 35-minute piece is in turn broken up into 3-6 scenes, with Motion template intros/outros and preselected music. After editing, the pieces have to compressed for the web and delivered via FTP.
So pretty much cookie cutter. How much time would you budget for this?
I want to come up with a number that's reasonable--for me and for the client--and if they accept my proposal, I'll do one of these batches of pieces every month of so throughout the year.
A bare minimum rule of thumb could be twice the duration of the expected running time. More comfortable to me is 3x or 4x the running time. Just as a ballpark figure, assuming all you have to do is simple cuts and dissolves and dropping in the occasional lower third or graphic.
Always add in a fudge factor for the kinds of things you mentioned that can pop up. The moer experienced you get and the more prepared the client and material are, the more you can shave that estimated margin.