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

The Destiny 2 Wiki says 12 seconds of continuous combat triggers it for a 15% damage bonus; combat starts on dealing or taking damage. The buff lasts 5 seconds per that wiki, but a search summary of another database gave 7.2 seconds and a 5-second combat window; sources disagree, so check in game. Dealing or taking damage refreshes it.