It depends on the knife, the sheath and you. A light knife in a low-profile sheath on a well-adjusted cord disappears within an hour. A heavy one, a bulky sheath, or a cord that lets it swing will annoy you all day. Weight and cord length are the two variables that decide it.
Weight, obviously. Under about three ounces including the sheath is where most people stop noticing it. And cord length: hung too low it swings and catches on things, too high it sits against the throat.
Mid-chest, above the sternum, is where most people settle.
A molded low-profile sheath with good retention beats a bulky one, and it must hold the knife when inverted and shaken. Test that before trusting it.
A breakaway cord is worth having if you work near machinery or livestock, for the obvious reason.
Some people never get used to something hanging against the chest, particularly under a fitted shirt. Others forget it entirely. There is no way to know except to try.
See what a neck knife is used for for whether the format suits your cutting at all.