if you're performing an Audit for a whole website, e-commerce or blog based site, how long do it takes to finish? from the on-page analysis to the keyword research down to linkbuilding. can you do the SEO Audit for one or two days?
How long is a piece of string? It takes as long as is required depending upon size and complexity of the site, keywords being targeted, etc. I guess you're unsure of what to tell clients, so pick a few sites (I assume you at least have some of your own as "examples" of SEO work?) of different types and sizes, do the audits and time yourself. As you do more audits, you will be able to refine the processes, find tasks you can automate (but don't rely upon it too much) and improve how you can record the results. Don't forget documentation - not just the results, but how to do each step, etc. You'll find yourself frequently fine-tuning it. In the end, once you've refined your approach, you will be able to get an idea of how long it will take you to do a site of x type and y size.