For work that I'm highly experienced in, like traditional art, graphic design, photo retouching and video editing, the job typically will go much quicker; those kind of jobs result in less time to complete and less cost to you.
As for web design, 3D modeling and visual effects, my skill levels are intermediate to semi advanced level; I still can produce quality work, but it may take longer. Each different job will vary in time, depending on the amount of work and details, which are things we can further discuss upon meeting for a consultation.
I keep track of and document every hour and minute I work on any project and always forward my hours spent and guestimated time left to my clients.