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Depends on how fast the clouds are moving, but generally I've found between 4-8 seconds works well. The videos are at 30fps, so you can work out how many frames you'd need a video x seconds long..
I used a cheap intervalometer to get the camera shooting while I went off and did something else. And LRtimelapse is what I used softwarewise (with Lightroom), its free for short sequences too.
A quick time-lapse from London. Really getting into these.