Apple IOS App icon title: maximum length?

It depends on the width of each character in the display name. Between 11 (for "wwwwwww…") and 15 (for "iiiii…").

Not only does this depend on the width of every character, but the device and iOS version also makes a difference. For instance:

the iPhone 4 till 5 will truncate names faster then an iPhone 5s with iOS 8 (while an iPhone 5s with iOS 7 will also truncate a long title). The iPhone 6 & 6 plus have a wider display but do show only 4 icons, which means more space for your title.

The best way to test this, is with the iOS Simulator.

So as an answer to your question: There is not a real limit as it depends on character length, device width and iOS version...