History of Frenzy

Frenzy is a weapon trait: being in combat for an extended time increases damage, Handling and reload speed until you leave combat. It was introduced in Season 13.

Numbers

Twelve seconds of continuous combat triggers it for a 15% damage bonus. Combat starts when you deal or take damage, and dealing or taking damage refreshes the buff. The Destiny 2 Wiki lists a 5 second buff. Another database lists 7.2 seconds and a 5 second combat window, so the reported durations differ.