I think this technique is more correctly called time laps and not stop motion. Stop motion is for creating claymation and stuff. There is an intresting linux app for it called frameworks (try watching the lego videos that guy did using this software.

And BTW, the URL should be of the jpg image output of the cam, directly. not the webpage of it.

When deciding the duration, test it yourself in the browser first. keep on refreshing and see how many seconds it takes to refresh. Ofcourse, if the webpage explicitly says how frequent the image get updated. Use this data.

Even though you can choose the duration according to your needs. But be aware that you will encode the video at 25 frames per second. Which means a 12 hour period will pass by in one second, if you decided to make the duration = 1800. This will not be smooth at all. However, if you take 1 shot every minute you will cover a 25 minutes period in one second. Which is very nice. Taking more frequent images, depend on what you need. Experiment and see.

The prague video above took 52M of jpg images space. The sequence covers aprox 20 hours period. One shot every minute. But the resulting video is just 5MB (RIFF (little-endian) data, AVI, 352 x 288, 25.00 fps, video: DivX 4) around 32 seconds. I think the resulting video can be even smaller, with the same quality. But I have no idea on how to do that.