Additional note - if you come back to your machine and find the render's done and don't know which limit it hit, you can usually find out by viewing the log and looking at the last few lines. Help >> Troubleshooting >> View Log File...
Iray periodically records the number of iterations and the percent that it's reached, so you can see if you hit the iteration limit or the quality limit. Similarly, it will report the total render time, so you can see about that as well. (Basically, all the stuff in the render status window - which vanishes when the render is done - is also recorded in the log file.)