When i do it i usually set my preview to 0 passes, that makes it go forever, you can just press pause if you want it to stop.
I also set my renders to 5k unless its complicated, in the case i go higher, ive done 50k at most though. The only way to knoe when the render will be clean is to set it high and check once in a while to see if it cleared up, if it didnt just let it continue, if it did, then stop it and save the image.
In cycles you should render in GPU mode if you have an nvidia card, im not sure how well the openCL drivers work with cycles. CPU mode is so slow
with my card(GTX680) i render real fast
It was way faster than CPU when i used GTS450, GTX560 and GTX580, so almost any new GPU thats in the top budget range or above renders faster than most CPUS.
If you share your file i could see how long it takes for it to render on my GPU, unless you own an nvidia card yourself.