I make financial and stocks audits. I have done audits for NGOs, accountants and lawyers. There can be several purposes for an audit such see is a logistic chain is working, see if a service is working, see if there are hidden expenses, see if there are financial leaks, see if there are merchandise loss, see is a schedule match with a costs, etc...
I work with data and field investigations. An audit usually lasts for several days and sometimes months if there is an investigation. People usually hire me to have a clear vision on a situation and find solutions.
When there is an investigation, I start with data analysis, then I present a plan of investigation and a schedule with estimated costs. If you have a long project and don't know how many days the audit may last, I recommend to start with one day to start estimate the full project.