UIButton Long Press Event

You can start off by creating and attaching the UILongPressGestureRecognizer instance to the button.

UILongPressGestureRecognizer *longPress = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(longPress:)];
[self.button addGestureRecognizer:longPress];
[longPress release];

And then implement the method that handles the gesture

- (void)longPress:(UILongPressGestureRecognizer*)gesture {
    if ( gesture.state == UIGestureRecognizerStateEnded ) {
         NSLog(@"Long Press");
    }
}

Now this would be the basic approach. You can also set the minimum duration of the press and how much error is tolerable. And also note that the method is called few times if you after recognizing the gesture so if you want to do something at the end of it, you will have to check its state and handle it.

As an alternative to the accepted answer, this can be done very easily in Xcode using Interface Builder.

Just drag a Long Press Gesture Recognizer from the Object Library and drop it on top of the button where you want the long press action.

Next, connect an Action from the Long Press Gesture Recognizer just added, to your view controller, selecting the sender to be of type UILongPressGestureRecognizer. In the code of that IBAction use this, which is very similar to the code suggested in the accepted answer:

In Objective-C:

if ( sender.state == UIGestureRecognizerStateEnded ) {
     // Do your stuff here
}

Or in Swift:

if sender.state == .Ended {
    // Do your stuff here
}

But I have to admit that after trying it, I prefer the suggestion made by @shengbinmeng as a comment to the accepted answer, which was to use:

In Objective-C:

if ( sender.state == UIGestureRecognizerStateBegan ) {
     // Do your stuff here
}

Or in Swift:

if sender.state == .Began {
    // Do your stuff here
}

The difference is that with Ended, you see the effect of the long press when you lift your finger. With Began, you see the effect of the long press as soon as the long press is caught by the system, even before you lift the finger off the screen.