What is the time limit for two taps to be considered a double-tap, on the iPhone OS?

// Edit: Why is this important?

In order to handle single-tap and double-tap differently, Apple's guide says to do performSelector...afterDelay with some 'reasonable' interval on first tap (and cancel it later if the second tap is detected).

The problem is that if the interval is too short (0.1), the single tap action will be performed even when double-tapping (if relying only on tapCount, that is). If it's too long (0.8), the user will be waiting unnecessarily for the single-tap to be recognized, when there is no possibility for a double-tap.

It has to be exactly the correct number, in order to work optimally, but definitely not smaller, or there's a chance for bugs (simultaneous single-tap and double-tap).