As seen
here:Notes: If a device doesn’t support vibrate (iPad, iPod Touch), nothing will happen. iOS devices may only vibrate for 2 seconds. In other words, you have no control over the duration.
I'm not that familiar with mobile, and I don't know if recent versions of iOS have that option. I did a quick search on stackoverflow and saw people mentioning that Apple doesn't like when you mess with the vibration duration, and having it less than their minimum length is not really an option (if you manage that you could have your app rejected, it seems).
I could be wrong though. Someone more knowledgeable could chime in.