It all depends on where you're taking the measurement. At the shooter's ear? 3 feet to the side? At the muzzle? Downrange?
SilencerCo claims that their Specwar 556, for example, meters at about 130dB, compared to an unsuppressed noise of about 165 dB. And it's not immediately clear where and how they're taking those measurements, but let's assume that they are comparable to each other.
The problem is that measuring noise levels is extremely tricky. Different equipment at different places on different days will give different readings for the same noise source. Plus, the dB scale is logarithmic, so the difference between 50 dB and 60 dB is not the same as the difference between 100 dB and 110 dB, so interpreting the results is tricky, even if you get it all straight. Plus, sounds have subjective qualities that typically measuring equipment won't be able to display, at least not in a way that's easily decipherable. So the answer to your question of "how many dB less" is not at all as straight forward as the "about 35 dB" answer that I gave you.
The ballistic crack is usually not the loudest part of the sound of a gunshot, especially not to the shooter because it's mostly occurring downrange. I will say, though, that .223/5.56 through a suppressor is probably not hearing safe, if you want a reference point. It will still ring your ears and be uncomfortable without hearing protection.
If you really want to have a lot of fun with an AR and a suppressor, 300BLK is where it's at. But that's a whole big thing.
