One second is enough if you're ready for it, but for the three engine failures I've had, I wasn't prepared for it - fortunately the cost so far has been only minimal airframe damage and wounded pride.
Doesn't this support the case for a technical solution to increase reaction times? We're all human and after years of flying without engine failures, we're no longer really ready for it (although we still think we are). Inevitably, some form of complacency will set in, I suppose.