The purpose of a website audit is to provide you with a full detailed outlook on your site health. A good and thorough website audit should be able to tell what what are the biggest problems for your website and what are the opportunities.
A comprehensive SEO website audit should be able to tell you:
A website audit generally takes around 4 to 8 weeks but mostly dependant on the size of the website. It is important to remember that you should not rush the process of a website audit for a comprehensive audit.