HI Philippe.
This is a different type of render engine, the render can last forever if you want, think about it like the shutter speed of a camera, the longer it is exposed, the most light enters in the sensor.
This is somewhat similar, you may define a fixed render time, let's say 10 minutes, and you will be sure that the render process will be finished in 10 minuts, but the image could be still very noisy, the other way to stop the render is to define a sampling level, for example 100 samples, this way you will ensure a minimum sampling level (a defined noise level), and this is the mode you should use for animations to avoide different noise levels between frames, now, how do you define the sampling level you want? you can leave the render for an undefined time and check the level from time to time, one you see you are in the level you want you know this is the stop level you should use, also you can think in time, try limiting the render in time, let's say 15 minutes, now if you are happy with that result check the level reached and use that, if you are not happy, repeat the proccess but increasing the time.
BTW sampling levels are passes in corona.
Hope this helps you a bit.
Cheers!