Every job i've ever gotten i was notified in under 24 hours. That being said i've heard stories of. Chances of getting an offer absolutely do not depend on the number of interviews.

Understanding the Context

A smaller company might have just 5 candidates in the final round, so a 20% chance would be fair to say. If you have a 10% chance in the final round then after 5 final rounds, you should have a. If it's for a permanent role, you may get a few days to a week to think about the job offer, if given.