In a one-on-one situation with no interference, 1-5 seconds is how long any skill testing, exciting FIRE-FIGHT should last that ranges from unbuffed light being gunned to probably a buffed heavy. Anything longer than that implies you are not completely engaged with the other guy or you are just terrible terrible aim lol.
Of course 1v1s are not going to be the norm in a team-based game, but as perspective. (opinion based on other SD games and videos)