How long can cat go without insulin?

I'd say take him to a vet and ask him/her. If your cat needs insulin, he/she has a serious condition that should not be taken lightly. If you cannot afford to give your cat insulin, take him/her to a NICE pound-like place and they will take care of kitty's medical needs. I've had this issue, but I cared more about my cat's life than having her live with me because I couldn't afford to take care of her.