However much your vet tells you for your kitty (you have to know a weight to know how much) and the problem. If you haven't seen a veterinarian how would you even know if penicillin is effective for whatever is wrong? Every antibiotic doesn't work on or is the recommended treatment for every issue. Different ones work on different things or not at all and if not you may cause more problems. Maybe the pen would react to the actual recommended treatment. Also, if you are asking how much you may not be experienced enough to use it and then will cause yet more problems. What if you cause necrosis (dying tissue) or something because it's given improperly? What if it's I'm and you give it wrong and now you have to amputate the leg? Abscess at injection site? Far more expensive than seeing your vet in the first place and learning the proper protocol or your kitty getting much worse because penn wasn't the correct antibiotic so now you have an emergency stay to try and save the kitty or something. You and particularly your kitty are probably far better off seeing the vet at this point if you don't have any idea of what your doing and may be cheaper in the long run. Hope your baby is feeling better soon.