haha, yea I was thinking 'holy shit thats deep' my first stab too. I bought 22 1.5's they work well but really half that would be fine. When I run out one day Ima reorder some 25 3/4" I think that would be perfect for injects. For some reason (it was years ago now) when I ordered pins I bought waaay too many.
Things to consider:
If you're fat you need a slightly longer pin, but even so a 1.5" is fine anywhere.
A standard 10 ml vial is about 2 inches deep so to get all that good stuff out you need a 2 inch pin. (Unless you have amps)
You should have a little more than 2x the needles of injects on hand in case of mistakes.
Remember to draw with 1 needle inject with another. (unless amps)
Wash hands and inject with soap and hot water and be generous with the rubbing alcohol!