I'm looking at my ancient Nord bow that I have and I see a green icon, I don't even know how to explain how it looks, and its in the top left corner of the bow's info box.

Anybody got a clue what this icon is?

Poison!

Oh, haha. I've never looked at my equipment poisoned. Thank you.

I had to jump in game to check. Neat little icon though. Never saw it before.

Now remember that it's your bow that gets poisoned if you apply a poison-potion, not the arrow!
It's up to you to decide, which type of arrow you want to use for the poisoned shot....a cheap steel arrow or expensive glass- or daedric arrow.
And it's 1 shot/poison-potion.

I have so many poison-potions, that I usually keep my bow poisoned all the time, and pick the arrow for the shot depending on the situation.
Also, if your bow is enchanted, the poison will stack to this effect...a bow with fire-damage enchantment becomes extra powerfull (for that particular shot), if you poison it with weakness-to-fire, for example.

If you take the "Concentrated Poison" perk from the alchemy tree, your potion will last 2 shots!