In the Iray Render Progress Rendering Settings you can turn off limits for Maximum Samples (it defaults to 15000). Setting it to -1 and the Maximum Time to 0 seconds causes the Iray render to take as much time as it needs to get 100% convergence for the quality setting you’ve chosen. (I typically bump it up from 95% to 99%).
Not sure when DAZ changed the behavior of Max Samples. It used to allow you to set it to -1 about a year or so ago. Anyway, I was pleased to find I could turn off limits in the Parameter Settings just like with any other parameter.