Toss it if you re-use the needle. It's the needle that's been in a calf, and then back in the bottle that will ruin it. If the bottle only sees a new sterile needle, it should be OK. Make sure you don't drop it and get 'stuff' on the little rubber top. :? The bottle top needs to be absolutely uncontaminated.
Toss it if you don't keep it cold ALL THE TIME. That means out of the sun and in a cooler or better yet, the fridge, between syringe fillings.
We will use a partial bottle if we only have a couple of calves to vaccinate, but we take just the number of doses we need, put them in a syringe with a brand new needle, and leave the vaccine in the fridge in the house.