Weapon Focus is how long the breath holding stabilizes the weapon. At max level you get roughly an extra quarter of a second of held breath before the weapon sway resumes. Whoopee. Like most of the stat boosts from leveling, it makes no real gameplay difference.
As for how long it takes to max out a class, that depends entirely on how well you're scoring with it. Weapon levels are bestowed by kills, but the classes are leveled by points. Soak up those point bonuses from objective zones. The Honor 80 requirement took longer for me, I already had SL and Commander maxed out on the points requirement long before hitting the honor level requirement.