Intervals (time between shots) where do I start?
-The easiest 'one size fits all' way around intervals is to get them as 'tight' as possible. So if your exposure is under 1 second simply take a shot every second BUT with many cameras (as mentioned in the 'stills or video' section) can't purge the buffer quick enough (especially shooting RAW) so you'll have to make sure your buffer clears before taking the next shot. On the other hand many bodies can do like 3 frames per second JPG continuous if you simply lock down the exposure button (most interval timers have this function).
-On the other hand you're sucking up memory quickly by going as tight as possible and it can be overkill, so one approach is to shoot some sample frames and scroll review them on camera. This will give you quick feedback on the resulting footage so you can adjust... maybe longer intervals will be fine, maybe shorter will give you what you're looking for...
-Some ballpark interval starting points to try out...
Fast moving clouds: 1 second
Slow moving clouds: 10 seconds
Sun moving across a clear sky: (wide) 20-30 seconds
Stars moving across the sky: (wide) between 20-60 seconds
Sunsets close up: 1-2 seconds
Crowds of people: 1-2 seconds
Plants growing eg cucumber vines: 2 minutes
Shadows moving across the ground: 10-20 seconds
Note these times can change drastically depending on the local conditions, and the aesthetic look you are trying to achieve, so the best answer is to get out there and practice for yourself to see what works for you. As a general rule of thumb, any tighter or telephoto shots need much faster intervals.