Hey there! Some more recent models have an interval-mode implemented in the software, so you can choose and set your intervals in the camera. Professionals are using so called intervalometers which you can connect to your body and then choose all the settings. 20 seconds is quite a big interval, for most of the cloud shots I went for something between 5-10 sec. But then again it all depends on your prefered framerate and your overall vision of the final video.