The minimum time in practice depends on various factors which I cannot entirely predict and compensate for. Some of the factors are:
1. The kind of device you have.
2. The camera being used on iPhone 4 and the latest iPod Touch.
3. The size of the photos/video you are storing.
4. Other tasks the phone might be performing in the background.
For example, on iPhone 4, with the front camera, setting the period to 'Back to Back' and encoding a video only will yield a shot rate of 3-4 photos per second while using the rear camera is much slower.
The interval selection will try to stick to the setting and will not go lower but it may go higher if the device can't keep up.