system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] What level does a threat have to be before the Avengers step in?

I guess this is specifically pre-Civil War, but presumably Iron Man and War Machine were still somewhat active under the Avengers title since Tony was planning on inducting Spider-Man.

In all, we see the collective Avengers in 4 battles as a team before splitting up:  the Chitauri Invasion, the battle at Strucker's Castle, the Ultron fights, and the fight with Crossbones' terrorist group.  Out of those, only the Chitauri Invasion and the Ultron fights were potentially world-ending.  The battle at Strucker's Castle seemed to be more about cleaning up Shield's loose ends, but the fight against Crossbones could be considered either that or a general battle against terrorists.

What qualifies a threat as being Avengers-worthy?  Would Cap have slipped into a telephone booth to suit up if he saw a bank getting robbed on his day off?  Did they typically only deal with groups like Hydra?  How common would a fight against something like Abomination or the Maximoff twins be?
gpt
All the Avengers weren't even in Lagos for the fight with Rumlow.  That was just the team on duty at the time.     

They normally act like a police force or fire station, with members pulling shifts.   If you're on the clock when there's an alarm, you respond to it.  When you're working and there's no emergency, you're following up leads and tracking down criminals.  Otherwise you're off duty and living your life. 

Ultron and Thanos were four-alarm fires that brought out everyone on the roster.  And the assaults on HYDRA had everyone on deck too, when they needed the extra firepower.