Years ago we used to use an intervelometer with a film camera to get a time lapse, then, when video came along, we started rolling tape the entire time we wanted to film a TL. That was a long capture in an NLE as many TLs would go 20-30 minutes or more. Now we can use the video camera and record to cards which speeds up the ingest in the computer and makes for quick use on the time line. Some video cameras allow us to select how long an interval between frames for recording and will record the exact TL you want in the video camera. I prefer to use a DSLR for my timelapse and then record other scenes with the HD video camera of the area while the timelapse is running. It does mean another tripod on my daypack for the DSLR.
More recenty, still cameras with intervelometers or DSLR cameras have really allowed just about anyone to create stunning TLs for your productions. Planet Earth and many other productions have gone this route. Also, by using a DSLR, you are capturing more information than needed for an HD production so you can zoom or pan around on your time lapse in post production and add one more movement into your TL and project. That's sweet!
So, what's a good starting point for TL?
That's a bit arbitrary and really does come from experience but here are some starting tips. If you are filming a mountain range with clouds moving over it, the speed of the clouds determines the TL. If, viewing with the naked eye, they seem to have good movement over a peak, then setting a DSLR for 1 frame every 4-6 seconds and recording for 20 minutes or so should give a decent TL. Remember, if you are using 30 frames per second in your production, then 1 frame every 6 seconds equals 10 frames a minute so if you want your TL to last 6 seconds, you'll record 30 minutes. 6 frames x 30FPS = 180 frames divided by 6 sec = 30 minutes. Many times 1 frame every 5 seconds for 20-25 minutes yields a good TL. If the clouds are moving faster or slower, just adjust the frames per minute or lenthen the record time. Faster clouds means more frames per minute and less record times. Slower clouds means less frames per minute and longer record times to get the TL. The closer you are or the farther away from the mountains also influence the TL.
After 4 or 5 TLs have been recorded, you'll have a pretty good eye and be able to judge and get close. If you're not exactly sure or the cloud movement looks slower after you start, just let the TL go longer to be safe.