Iray, unlike Luxrender last time I used it (year ago or so), has some convergence criteria to terminate the render. Iray renders will not run forever by default. The main quality control is done through the 'Rendering Converged Ratio'. This is some measure of what percentage of pixels have converged, ie value of pixel is not changing within some tolerance with each iteration. The default is 95%. If you get to 95% convergence that produces a pretty high quality image. Some people have gone for 99% convergence, but I don't see any rational for that in anything i've rendered. Low convergence, like 20-50% might be a draft render.

There are several other controls that will also stop the render, even though it has not reached the convergence target.

Max Time (sec) - the default value on this is 7200, 2 hours. If you leave it at that render will end at 2 hours even it is has not reached the convergence you want. The Maximum value is 259,200 which is 72 hours, but I think you can set it to zero and it will run forever. This is the maximum time. It will quit if it reaches the convergence target before this.

Max Samples - I think this is the max samples for a pixel. If you reach this the render strops even if the convergence target is not reached. Default is 5000. Maximum value is 15,000. I think you can turn off the limit and set higher values or use 0 which makes it unlimited. Again if it reaches convergence target or any other max limit is will quit first.

Render Quality - This apparently effects how Iray considers a pixel to be converged. The default is 1. I have always just use the default and I have seen posts indicating you should not fiddle with this. I know some people use values like 1.5 or 2, but i would just leave this alone.

So what max time or max samples do you need to set for a given quality? That is totally dependent on what is in the scene and how the lighting is done. Iray render times are very sensitive to lighting. The best thing is to just set huge times and sample or maybe zero (i have not tried setting zero), so neither the time or samples stop the render and you can judge based on quality you see if you want to stop before reaching the convergence target you set.

Iray likes outdoor scenes using the sun/sky or a good HDRI on the sky dome. In my experience, indoor scenes are a lot slower to render.