Some sample audits or those looking at specific elements may be carried out within a day (although it could be argued that these aren’t actually audits), whereas in-depth auditing of large scale sites can run into several weeks.
There are some occasions where a set budget may be available. In these instances the process can be flipped on its head. If you know the budget, and how much resource/time that equates to, then it becomes a matter of assessing where the time would be best spent to return the most value.
It’s also important to recognise the difference between the resource it takes to carry out an audit, and turnaround time. For instance, an audit might be priced based on an estimate of 3 days of resource, but the turnaround time may be several times that due to scheduling, technical dependencies and individual preferences to ways of working. This may seem like common sense, but when dealing with clients or other stakeholders it’s critical that these sort of expectations are managed.