The brief answer provided by
psyco75 is better than what I could have written.
I will just add:
I have a good graphics card, so the render time itself can be between perhaps 10 minutes and 90 minutes, only.
But what can take even a couple of hours (or more) is the preparation of the scene for render.
Composition, surface materials, lights, d-force simulation, posing of the models, adjusting of every finger bend, and frequently, toe positions so that they fit within the shoes, etc, etc...
And then you view the scene in iray, and you see things that can be improved, so you are back to readjustment.
I need to stop now, just thinking about it is giving me a headache.
In the next few days I will do a render that takes a relatively short time to do.
But here are a couple that took hours to complete. In particular, the render time was long because of many lights and reflections.