Let's see... You'll be hitting zeros and stuff. From 1 to 70 takes 737627 xp. If you used 70k arrows, you'd need to hit an average of 3 points per arrow. Hmm... With 100k arrows, you'd need to hit an average of 2 per arrow. I'd go with 100k arrows to be safe. According to Tip.it's Ranged Damage table, you won't hit 3's until you hit lvl 12, and you won't hit 4's until 21, so it's better to need an average of 2 points of damage per arrow.