Minimum 10 years to come on stream. By then the current noisiest aircraft such as the 744 will have gone. So will most 73 Classics, 767s, MD80s etc - there will be a noticeable reduction in per aircraft noise. 10 years after that, aircraft noise will be a non event - probably won't hear A320/737 replacements beyond the perimeter and widebodies already dominated by 787/A350 which will be much much quieter than the already quiet A380 will be on their way to being replaced by whatever comes next which is bound to be quieter still. There may be other issues but, looking ahead, noise shouldn't be one of them.