It depends on what? Some clients come to me to actually solve the marketing problem, concept, write and design the motion graphic.
1. Concept work, motion graphic idea (2-3 options will be presented in comp form, client chooses one) 2-3 hours
2. Client chooses one and refine based on feedback. (Revisions shown in comp form) 1-2 hours, repeat as necessary, 1-2 hours per revision cycle, depending on nature of revisions requested.
3. Upon approval, create animation. (typically a 600x450, and the hours can vary depending on complexity of animation) 2-4 hours
4. Revisions to final. approx 1 hour, depending on nature of revisions
5. Resizing and back up gifs/jpgs. I typically estimate .25-.5 hour per resize, unless some of the resizes are so tiny that I literally have to recreate the message and design to get something that communicates the idea.
An estimate like this allows you to give a range, allows for some wiggle room. You can also point out to the client that more than one revision per stage is okay but it will cost them.
But whatever you do, don't give ANY estimate without first discussing with the client what they want and then sitting down to really work out what it will take in terms of hours to complete the project.