First, consider the scope of the audit. Will you audit one or more processes? Will you audit one or more departments or teams?
Then, study the audit criteria (ISO 9001:2015 clauses applicable within the scope, procedures, instructions, …). While studying the audit criteria start listing the questions that you want to make to whom, start listing what you want to see and where to think about the sample size that you should consider to later support your audit conclusions.
After this, you should have an idea about the time needed for each stage in your audit, considering the number of questions to ask and to whom, what you want to observe, the sample size, the number of records you want to check. Normally, 20 minutes is the minimum I take for each stage, and 1 hour the maximum.