Most OW airlines, BA included: 2-3 minutes before boarding begins or right at the tail end of the process. As I almost always fly J, there’s not much advantage to being super early.

AA: 20 minutes before boarding is scheduled to begin. I’ll definitely never risk arriving to the gate right as boarding is scheduled to end. All it took was a single flight where boarding started and ended 30 minutes early for me to realize that AA’s D-0 obsession is fundamentally anti-consumer. AA (specifically, their hyper-empowered gate agents) will happily screw me if I am not proactive and ensure I always arrive within the gate agent’s self-imposed, never publicly disclosed boarding window.