Specifically, it's something like "20 minutes per (on-foot NPC) murder, 2h per notoriety point, displayed rounded up to full notoriety points".2 hours in-game time per Notoriety unit for it to go. I usually go to a mat farming hotspot and wait for it to go.
So:
1 murder
=> 20 minutes of notoriety.
=> You will see "Notoriety: 1", because 20 min / 2 h = 1/6, rounds up to 1
=> The notoriety point will go away in 20 minutes.
3 murders
=> 60 minutes of notoriety.
=> You will see "Notoriety: 1", because 60 min / 2 h = 1/2, rounds up to 1
=> The notoriety point will go away in 60 minutes.
10 murders
=> 200 minutes of notoriety.
=> You will see "Notoriety: 2", because 200 min / 2 h = 1.67, rounds up to 2
=> The notoriety will drop from 2 to 1 point after 80 minutes, because 200 min = 2h + 80min, then it will take full two hours to lose the last point of notoriety.
I'm not fully certain if it's exactly 20 minutes per murder, it's a number I found in a comment on the ED's wiki page for notoriety and somewhere on reddit as well, but the way each kill amounts to a certain amount of notoriety minutes perfectly matched my observations, so I'm pretty sure of the mechanism at least.