I use the same one for the morning and the evening shot, but no more than that, everyday is a fresh needle. (But I also use a triple bevelled animal insulin needle as oppose to the single bevel human ones). If you reuse them too much they lose the coating that makes them slip in and start to hurt. If you were to look at them under a microscope you would see that they start to get rough. And as cheap as syringes are there really isn't any real reason to use them for several days and hurt your kitty.
Mel, Maxwell, Musette & The Fur Gang