The comment is made elsewhere that archers didn't collect arrows during a battle:
I agree with Felix Goldberg, the archers most probably didn't reuse the arrows in a single - unbroken - battle, since they had to keep formation and received orders. They might pick up the arrows only if it is there, pinned into the dirt and healthy (like they were targeted by enemy archers), but this is unlikely too, archers typically used against footmen and cavalry.
However, this source for the Battle of Crecy explicitly states (my emphasis)
Each successive charge was weaker and during brief pauses in the battle, the English archers stood in their lines with remarkable discipline, only going down the slope far enough to collect their arrows.
I have seen this and similar comments made elsewhere, though this is the only source I can locate just now. It is important to remember that each of the reported charges by the French knights only lasted a few minutes, say 5 or 6 at the outside, as any charge lasting longer has lost it's most important advantages, speed and momentum.
Each of these charges would have required a much longer time period, perhaps 20 or 30 minutes, for the participants to rally, form up in units, and move to their respective start zones. During these periods there was ample time for designated individuals to run forward and collect arrows.
A quick look at the attached map of the battlefield for Crecy makes clear that the launch zone for the French knights was located 3 or 4 times effective arrow range from the English lines, so the question of safety for archers running forward really doesn't exist.
My understanding is also that the archers and runners running forward to collect arrows had knives and daggers which could also be used to kill any wounded enemies who attempted to resist such endeavours.
So while a specific value for the percentage of arrows that can be reused during a battle is unavailable, some simple calculations regarding maximum troop coverage of ground during a charge (at the trot/canter and then gallop, coupled with the inherent inaccuracy of bows used essentially as artillery, suggests that perhaps 90% of arrows fired fell harmlessly to the ground (or deflected with minimal damage from armour)and that most of these could be reused as soon as collected.