Corona will render indefinitely unless you specify a parameter that when reached will stop the rendering process. Those parameters are found under the Render settings -> Scene menu. In there they are called "Pass limit", "Time limit" and "Noise limit".
You can set any of the above parameters to your liking and that'll make Corona stop rendering.
But why is Corona rendering indefinitely? Well, Corona is a progressive renderer which means that each rendering "pass" adds more pixel information on top of the existing image. Basically, with each pass the image will be cleaner and less noisy because it will be better sampled.
So what if you stop rendering too soon? Well, the image will be still be "realistic" but it will be noisy. If you stop it real soon (at pass 1) in an interior it might be incredibly noisy. Leave it rendering for 50 more passes and you'll probably end up with a noise free image. No matter how much noise you have though, the realism will be there.
Probably the easiest parameter to control when it comes to stopping the rendering process is the noise limit. Typically you'll probably end up with a value of around 2-4% for most of your renders. In a studio environment it might be lower but in an interior it might be higher. Personally, because I always do test renders before I send something out, I just have it set to 3% and then if I think the image is still noisy I just render at 2% for the final image. From my experience in most projects you won't go below 1,5% or higher than 4%.
If you want to you can also set it to stop rendering after it does an X amount of passes. Typically, after using Corona for a while you'll probably have some sort of an idea of how many passes you need for a given scene but ultimately, you'll still probably do test renders in smaller resolution just in case. Alternatively, you can also just use a time limit.
Hope thats at least somewhat helpful :)
