Ad aspera

Aren't percentage-based bonuses useless for ranged accuracy?

Yes.

How long does this buff last?

Until the end of the battle.
Non-physical bonus damage from weapons is not multiplied by anything, not even critical hit bonuses.

Added 1 minute ago
I made a mistake with "cold." The word "percentage" was unnecessary.

Added 1 minute ago
You can buff any ally with any weapon from any distance.