The UA Oath of Treachery paladin (from Unearthed Arcana: Paladin) gets the Aura of Treachery feature at 7th level, which gives them this benefit (in addition to another one, Treacherous Strike):
Cull the Herd. You have advantage on melee attack rolls against any creature that has one or more of its allies within 5 feet of it.
It does not specify for how long this 'advantage' lasts. Its wording, unlike Treacherous Strike, does not imply any exact limit on the number of times that one may utilise this ability – only a condition in which the advantage takes place.
The "you may use this ability up to three times per short rest" that comes immediately after Treacherous Strike also seems to support that Treacherous Strike is, by RAW, limited in its usage, which is further reinforced by Treacherous Strike's own text.
Thus I believe that Cull the Herd doesn't have a limit on the number of uses it has by RAW, but am wondering whether or not this interpretation is correct.
Best Answer
Cull the Herd applies to every attack as long as the conditions are met
The ability doesn't specify how long the advantage lasts because it doesn't have a duration or a limited number of uses. Each time you make an attack, if the conditions are met at the instant you make the attack, then the ability applies and you have advantage for that attack.