It doesn't seem to exist any method or function that do this directly, you must take the difference of the two points coordinates and use pythagorean theorem:
CGPoint touchBegan;
CGPoint touchEnd;
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
UITouch* touch= [touches anyObject];
touchBegan= [touch locationInView: self.view];
}
- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch* touch= [touches anyObject];
touchEnd= [touch locationInView: self.view];
CGFloat dx= touchBegan.x - touchEnd.x;
CGFloat dy= touchBegan.y - touchEnd.y;
CGFloat distance= sqrt(dx*dx + dy*dy);
< Do stuff with distance >
}