Listen to all touch events in an iOS app

A subclass of UIWindow could be used to do this, by overriding hitTest:. Then in the XIB of your main window, there is an object usually simply called Window. Click that, then on the right in the Utilities pane go to the Identities (Alt-Command-3). In the Class text field, enter the name of your UIWindow subclass.

MyWindow.h

@interface MyWindow : UIWindow
@end

MyWindow.m

@implementation MyWindow

- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event
{
    UIView *res;

    res = [super hitTest:point withEvent:event];

    // Setup/reset your timer or whatever your want to do.
    // This method will be called for every touch down,
    // but not for subsequent events like swiping/dragging.
    // Still, might be good enough if you want to measure
    // in minutes.

    return res;
}   

@end

You need a subclass of UIApplication (let's call it MyApplication).

You modify your main.m to use it:

return UIApplicationMain(argc, argv, @"MyApplication", @"MyApplicationDelegate");

And you override the method [MyApplication sendEvent:]:

- (void)sendEvent:(UIEvent*)event {
    //handle the event (you will probably just reset a timer)

    [super sendEvent:event];
}

You can use a tap gesture recognizer for this. Subclass UITapGestureRecognizer and import <UIKit/UIGestureRecognizerSubclass.h>. This defines touchesBegan:, touchesMoved:, touchesEnded: and touchesCancelled:. Put your touch-handling code in the appropriate methods.

Instantiate the gesture recognizer in application:didFinishLaunchingWithOptions: and add it to UIWindow. Set cancelsTouchesInView to NO and it'll pass all touches through transparently.

Credit: this post.